Officers Tasered Williams - who was only released from prison two weeks after being jailed for attacking his ex-partner - and he became unresponsive. He later died in police custody.
Today the probation service said it would be launching an inquiry after it emerged Williams was not being monitored despite being released from prison just two weeks ago following a five-year sentence for an attack on his ex-girlfriend.
It was also revealed that officers who attended the hotel following the attack are receiving counselling following the 'extremely tragic incident'.
Friends told today how Miss Yemm only met Williams hours prior to the brutal killing, after he offered to get her a taxi home following a night out.
They said the Next shop assistant must have been persuaded to go up to Williams' hotel room - at the B&B which was being used as temporary accommodation for homeless people - before he launched the horrific attack.
A friend of the pair said: 'They met that night in a bar and got chatting for the first time.
'They had been drinking and he persuaded him back to the hotel and said he'd get her a taxi home.
'Once he got her back in the hotel he must have invited her up to his room and that's where he attacked her.'
Miss Yemm's heartbroken mother Paula, 49, was being comforted at her detached home in Oakdale near Blackwood, south Wales, today.
Her brother Cavan, 25, was in tears as he opened the door of the family home - a smart detached house on sleepy street. Her sister Shannon said: 'We just need to be left alone to grieve'.
A police family liaison officer was also at the property.
Relatives of Williams said they 'were in complete shock' following the death of an 'innocent' woman.
In a statement, his family said:'We are devastated by the death of an innocent young lady.
'We are in complete shock. Our thoughts and prayers at this time are with the victim her family and friends. Words cannot express our grief.'
Police have been trying to piece together the last hours of Miss Yemm's life and officers have been carrying out forensic tests on a silver BMW believed to belong to Williams - who was from the village of Blackwood and nicknamed Fifi.
Officers are trying to work out whether he drove her back to the hotel in the sleepy village of Argoed, where he was found 'eating her to death while high on cocaine'.
Lyn Beasley, who lives close to the hotel, said: 'He went Hannibal Lecter on the woman, he gouged her eyeball out, ate them and ate half her face.
'He had just been released from prison and was high on coke.'
And Jill Edwards, who also lives near the hotel, said: 'This animal was eating this girl to death. He murdered her so police stopped him - good on them.
'Security said they told him no girls in his room and he didn't answer, when they opened his door he was eating her face.'
Miss Yemm, who worked in sales at clothing giant Next, was described by friends today as a fun-loving girl who had been at a Halloween party with friends just days before the gruesome attack.
She was a huge fan of TV series Vampire Diaries and that night she had dressed as a zombie with friends.
She used the online nickname 'Yummy Yemmy' and knew of Williams through mutual friends.
Williams was described as 'an infamous character' in his hometown of Blackwood with a reputation for violence.
He had only just been released from prison after serving half of a five-year sentence for attacking a former girlfriend.
Another former partner, Emma Thomas, 38, revealed today that she was still under court protection from Williams - despite the fact they split up two years ago.
She said: 'He made my life hell. I've had nothing to do with him since we broke up more than two years ago.
'It is a terrible shock and I'm very sad that this has happened.'
She said the police 'had a lot to answer for' after Williams brutally attacked Miss Yemm.
'I'm in shock, I can't believe it has happened,' she said.
'Me and my family are thinking of them. At the end of the day I hope that the girl's family gets justice for this because the police have failed big time.
'My heart goes out to that poor girl and her family, it's just awful.
'He got out of prison two weeks ago. I'm still under protection from him, he has killed a girl. The police have a lot to answer for.
'I'm just mad at the fact that it is the police's fault because they could have protected that young girl.
'They know he was not safe.'
Despite his recent release from prison, Williams was not on bail or subject to any probation meetings.
The Wales Probation Office today confirmed a serious further offence review would be carried out because the offence had occurred within a 30 day period of his sentence ending.
Miss Yemm and Williams were seen drinking together shortly before the attack in the early hours of Thursday and Williams was described as 'seeming sober'.
However, he is thought to have taken cocaine before the attack in which he ate her eyeball and chewed half of her face.
Shelly Davies, a friend of Miss Yemm, said: 'I just keep thinking of how frightening and painful it would have been for her
'I feel sick to my stomach, it's horrific to think that one person can inflict some much brutality onto another.
Miss Yemm's father (pictured) and the rest of her family were being supported by family liaison officers at their family home in south Wales today
'I had bad dreams all night and I feel exhausted.'
Another friend, Kathy Young, said: 'My thoughts are with her family.
'No one should have to go through this, this is horror movie stuff. Not happening in your hometown.'
Gareth Griffiths, who lives in Argoed, said: 'We hear on the news that it has been cannibalism.
'Nobody wants to hear that, it must be a horrific way to die, it's horrible, it's unbearable.'
A friend of Ms Yemm, who did not want to be named, added: 'I feel stunned, shocked and sick to my stomach. It's horrific. She was a lovely person. She didn't deserve to die like that.'
Miss Yemm had been a pupil at Oakdale Comprehensive School and had studied Health and Social Care at Crosskeys College.
One businessman in the former mining village of Oakdale said: 'Everyone here knew Cerys - we watched her growing up.
'She was a lovely girl, smart, attractive and brought up by a very good family who she was extremely close to.
'They're nice people - they must be going through hell.'
A neighbour added: 'I can't bring myself to think about what happened to her, she was so nice, she would always say hello.'
Miss Yemm had seen her own former boyfriend Mathew Hayman jailed after breaking up with him in October last year.
Hayman, then 27, tried to kill himself after they split by setting his house on fire.
He lit up his bathroom before turning on the gas cooker - and it was 'pure chance' his house in Abertillery did not explode.
Hayman admitted arson when he appeared at Newport Crown Court and was jailed for three and a half years.
Police are continuing to investigate the double death at the hotel and the 'Hannibal Lecter' attack has been met with revulsion in the village.
A security guard at the hotel is believed to have rung 999 to say he had seen the killer eating the woman.
The officers who responded to a 999 call are being supported by Gwent Police's Force Occupational Health Department.
Gwent Police confirmed a murder investigation had been launched into Miss Yemm's death and confirmed a Taser was discharged on Williams. He later died at the scene despite police trying to save him.
Officers refused to comment on the specifics of the incident but sources confirmed Miss Yemm received substantial facial injuries.
A post mortem is currently being carried out on Miss Yemm but police said it 'may not be complete for some time'.
A Gwent Police spokeswoman added: 'We were called by a concerned member of the public to an incident at the Sirhowy Arms Hotel in Argoed, Blackwood where it was reported that a man was attacking a woman.
'On arrival both the man and woman were still at the location.
'A Taser was discharged, and a 34 year old local man was arrested. Whilst under arrest, the man became unresponsive. Officers and paramedics administered First Aid but he was pronounced dead at the scene.
'The woman who sadly died from her injuries at the scene can now be named as Cerys Yemm, aged 22 from Blackwood.
'Cerys' family are currently being supported by specialist Gwent Police officers.
'The murder investigation is continuing today, officers and Crime Scene Investigators remain at the scene. To confirm, we are not looking for anyone else in connection with the incident.
'We are cooperating fully with the IPCC, which is independently investigating the death of the man.
'At this time our thoughts are with the families of the deceased following this extremely tragic incident.
'Our officers who attended the incident yesterday are being fully supported by our Force Occupational Health Department.
'The post mortem for Cerys will start today, although may not be complete for some time.
'There is an increased police presence in the Argoed area at this time to reassure local residents. If anyone has concerns we would encourage them to speak to local officers.'
The force also confirmed it was not looking for anyone else in connection with the incident.
Speaking to reporters today, Chief Inspector Paul Staniforth said: 'The post-mortem for Cerys will start today, but will take some time.
'Speculation about what happened at the scene and the cause of death is unhelpful and very upsetting for the family of the deceased.'
The Independent Police Complaints Commission is also investigating Williams' death because he was in police custody after being Tasered and arrested.
A spokeswoman for the watchdog said: 'The IPCC is independently investigating an incident overnight in Blackwood in which a man and a woman have died.
'Gwent Police referred the incident to the IPCC early this morning.'
A Home Office pathologist and scenes of crime officers were at the hotel today trying to discover how the couple died.
Witnesses reported seeing forensic officers in white overalls at the scene, as well as a police cordon which blocked access to the hotel as well as the local Baptist church's graveyard.
Police were also seen carrying out forensic tests on a silver BMW believed to belong to Williams which was parked outside the hotel.
Local resident Cindy Farr, 41, told MailOnline of a 'significant' police response, adding: 'You do have police going back and forth regularly but I've never seen any problems. It is quite out of character.'
Grandmother Susan Gibbs, 71, who lives near the hotel, said: 'I'm not really surprised that something like this has happened. Police seem to be here every other night to sort out the violence.
'This was a quiet peaceful village before it was opened, now it's got to the stage where our houses are being devalued because of the problem. We've even had people sleeping rough overnight in the hedge rows.
'Once of the bodies was brought out wrapped in a brown bag on a stretcher.'
June Trace, secretary of Argoed Baptist Church, said the building had a 'fluctuating' population of people, mostly in their 30s or 40s.
A villager, who did not want to be named, said: 'Ever since [the hostel] opened, we've had constant problems.
'I've got a young family and I'm frightened to even walk near the place.'
Another resident said: 'The police are here nearly every other day.'
And Reverend Charlotte Moss of Argoed Baptist Church told MailOnline: 'We're doing what we can. All we can do at the moment is we've opened the building and made it available for people.'
She said she had been 'back and forth' to the church today - and added that it was there to support local residents, describing the area as 'a typical Valleys village'.
Welsh Ambulance Services were called by police at 1.30am and said they 'responded with a paramedic in a rapid response vehicle and a crew in an emergency ambulance'.
A spokesman added that a man and woman were both pronounced dead at the scene.
The hotel - located in the Sirhowy Valley in a village of about 2,500 people - has nine guest rooms, including three singles, four twins, one double and one family.
Local media reported five years ago how the building was being used as a bed and breakfast helping the homeless, having closed as a pub and hotel in 2008.
Local councillor and Caerphilly Council deputy mayor Leon Gardiner, 81, said Argoed had been 'hit for six' by the deaths.
He said: 'When I went for my paper this morning, I could see the road was closed off and there were police everywhere. When I heard that two people had died, I couldn't believe it.
'Everyone is totally stunned by what has happened. It's a shock because Argoed is a lovely place to live and I've never known anything like this to happen here before.'
Mr Gardiner added that he also felt sorry for the owners the Sirhowy Arms Hotel.
'They are a lovely couple and well liked' he said. 'They've done a smashing job - especially with the renovations in the past couple of years.
'It's a B&B but has been used as a hostel for homeless people. It's run magnificently.'
That sentiment was also echoed by a neighbour of the hotel, who said: 'We've never had any problems. The people who live there are not allowed to drink or smoke in the hotel so they go down to the woods to do it. Sometimes there are police vans outside if there has been a fight or something, but despite that it's still a quiet street.'
A spokesman for Caerphilly County Borough Council said: 'The authority is fully co-operating with Gwent Police as part of their ongoing investigations.
'We can confirm that the B&B establishment is used by the council for the temporary placement of single homeless people and has been in operation for a number of years.'
- Daily Mail
