Neighbours say Abedi, who studied business and management at Salford University before dropping out, had grown a beard in the last 12 months and had begun acting strangely.
"A couple of months ago he [Salman] was chanting the first kalma [Islamic prayer] really loudly in the street. He was chanting in Arabic," Lina Ahmed told The Sun.
"He was saying 'There is only one God and the prophet Mohammed is his messenger'."
Another neighbour, Leon Hall, said the young man "had an attitude problem."
One member of Manchester's Libyan community told the Guardian. "He was such a quiet boy, always very respectful towards me. His brother Ismail is outgoing, but Salman was very quiet. He is such an unlikely person to have done this."
Salman and his brother Ismail worshipped at Didsbury mosque, where their father, who is known as Abu Ismail within the community, is a well-known figure. "He used to do the five and call the adhan. He has an absolutely beautiful voice. And his boys learned the Qur'an by heart.
"Abu Ismail will be terribly distraught. He was always very confrontational with jihadi ideology, and this Isis thing isn't even jihad, it's criminality. The family will be devastated."
British Prime Minister Theresa May said Abedi deliberately chose to detonate an explosive at the end of the show to inflict "maximum carnage."
Ismail is believed to be in Tripoli, Libya, while his wife, Samia, is thought to be in Manchester.
A family friend revealed Ismail often comes and goes between Manchester and Tripoli but believes Salam would have been redicalised in England and not Libya.
"I can't believe [Salman Abedi] would have been radicalised in Tripoli. All those types have been driven out of the city. It must have happened here.
"But what was he doing, murdering all those people. There must have been somebody influencing him. It's terrible. He was off his head."
Despite claims Abedi was a quiet and resectful member of the community, one senior figure of the Didsbury Mosque and Islamic Centre said the 23-year-old gave off a different persona.
Mohammed Saeed told the Guardian Abedi looked at him "with hate" after he gave a sermon criticising ISIS.
He claimed Abedi signed a petition criticising him and his sermon against terrorism.
Saeed said a friend was so worried that he got his adult to sit next to Abedi in case the 23-year-old attacked.
Islamic State has claimed responsibility for the bombing, but Chief Constable Ian Hopkins said it was not clear yet how many other people were involved in the planning.
"Our priority, along with the police counterterrorist network and our security partners, is to continue to establish whether he was acting alone or working as part of a wider network," he said Tuesday afternoon.
Theresa May said the attack was among "the worst terrorist incidents" in UK history.
"A single terrorist detonated his improvised explosive device near one of the exits of the venue, deliberately choosing the time and place to cause maximum carnage and to kill and injure indiscriminately," she said outside her official residence at 10 Downing St, London.
