Police said yesterday they were investigating the circumstances surrounding the death and had arrested a 30-year-old man in connection with the incident over "concerns for the care of the baby at the time of the incident". He remained in custody yesterday evening.
One neighbour suggested the dog may have been jealous of the arrival of the new-born baby into the household.
The family are thought to have owned the dog for at least a year and it had become well known in the street for regularly escaping from the garden. Mr Young, a builder, would come out of the house on a leafy road in Sunderland to retrieve the pet.
Patterdales, although small, have a reputation as highly energetic dogs and were originally bred for hunting and killing foxes in the Lake District.
Neighbours yesterday spoke of their sadness and shock.
"They are a lovely family, they are a caring couple in their thirties. This is a complete tragedy," said Paul Tweddle, 50, a criminal psychologist, who lives opposite. "They had only brought Reggie home a few weeks ago. They just got on with their lives and were a normal family.
"I see the terrier that they have from time to time. Sometimes I see it clambering at the fence and it gets out. I shout to Ryan 'The dog has got out' and he comes and gets it.
"It is a tiny dog and I cannot say whether or not it is dangerous. Sometimes appearances can be deceptive it is impossible to tell."
He added: "It is a complete tragedy and I am so shocked. It should be such a happy time with a new baby. It will take the family a long, long time to recover from this."
Another neighbour Denise Haley, 59, said: "Maria was in the house a few weeks ago with Reggie. Maria is a lovely girl and I simply do not know how she will cope with this tragedy.
"I have seen the dog, it has got out a few times. Ryan shouts for it and it comes back in.
"I am scared of any dog but that dog seemed okay. It is a tiny dog and brown in colour.
"I am in total shock, me and my daughter have been really upset about it this morning. They are lovely people and they don't deserve this. I cannot imagine what she is going through, I hope she is alright and she has a lot of support."
Her daughter Rachel McLaran, 37, a close friend of Maria, said: "I Just feel really devastated for Maria. I can't imagine what she is going through. I've just had a baby myself so it is terrible. I fell sick about it."
Another neighbour, who did not wish to be named, said: "The dog is a Patterdale Terrier. The family tend to keep it in the garden.
"It would get out from time to time and I would pick it up and throw it back in. It was no bother at all.
"I know terriers very well as I have owned them before. I think the dog had become jealous of the baby and that is why this has happened.
"I know they have never had any bother with the dog in the past. It was Ryan's dog as far as I'm aware. He is a lovely lad, a hard working lad and an excellent father.
"He will be devastated by what has happened but I know he will have had no part in it. They are a lovely family and I hope they can get over this."
Northumbria police were called to the family's semi-detached home in the middle of the night after a report that a baby had been bitten by a dog.
In a statement, police said: "At 4.15am this morning, Saturday, June 20, police were informed a baby had been bitten by a dog inside an address in Falkland Road in Sunderland. The three week old baby boy was taken to hospital where he later died.
"A 30-year-old man who was inside the property at the time of the incident has been arrested and is in police custody helping with enquiries.
"The family dog, a small terrier, is currently in kennels and is likely to be destroyed. The dog poses no threat to the wider public."
Superintendent Gillian Mitchell, of Northumbria Police, said: "At approximately 4.15 this morning we received a call to say that a small child had received a dog bite at a home in the Sunderland area.
"Officers attended and found a three-week-old baby boy who appeared to have sustained a dog bite.
"Paramedics were called, they arrived very quickly and took the child to hospital.
"Unfortunately despite all their best efforts the child has died.
"Back at the house, there was a small terrier-type dog. That dog has been seized and has now been kenneled pending our enquiries.
"Obviously we have concerns about the care of the child at the time of this incident.
"I can understand the community will be devastated by this isolated but tragic incident where we have had a child so young die.
"It is important we remember the family and that they are supported. The police will stay with the family during the investigation and keep them up to date with what is going on."
A police car was parked outside the property yesterday while flowers have been laid outside the gate.
Social media sites were also inundated with condolences for the baby's parents. Pammie40 wrote: "I'm a dear friend of the family and it's absolutely devastating news. It was a tragic accident. We are all devastated. R.I.P little angel boy."
Ms Blacklin's grandfather Richard Blacklin, 75, said: "Maria is in a state of shock. The baby was only three weeks old. My wife died on Friday [the day before] from cancer. It's been terrible."
Ms Blacklin's aunt Sandra Blacklin said: "Maria's in a really bad way. He was a lovely little baby. I only lost my mum the day before that."
