A 23-year-old man with learning difficulties has died in hospital after being "savagely attacked" by a gang of up to five youths.
Mr Martin suffered a sustained and brutal attack
Brent Martin was found with multiple injuries on Sunderland's Town End Farm estate on Thursday night after reports of a man being beaten up.
He was taken to the city's Royal Hospital, where he died on Saturday.
A 21-year-old man and three youths, one aged 16 and two aged 17, are being questioned by police.
Mr Martin, described as vulnerable by police, had been the victim of a "sustained and brutal" beating.
Police have appealed to parents in the area to question their children about their whereabouts at the time of the attack.
Speaking on Friday, Det Supt Kay Blyth, who is leading the investigation, said: "I would appeal to parents in the area to think about what their children were doing last night.
"Did they come in late and were acting out of the ordinary? Did they fail to come home at all?"
She said the area where the attack happened had experienced low-level anti-social behaviour, but crime levels were relatively low.
Anyone with information about the incident should contact Northumbria Police.