A disabled man found dead in his home had been tied up, beaten and stabbed.
Police have described the murder of Keith Philpott, who had learning difficulties, as "sickening" and "extremely cowardly".
The 36-year-old's body was found in his flat in Axbridge Court in Billingham. His hands and feet had been bound.
The victim's family alerted police because they had not seen him for 24 hours. Cleveland officers had to force his door to get in on Thursday evening.
Det Supt Tony Hutchinson, who is leading the inquiry, said it was possible that Mr Philpott knew his killer.
He said: "Officers had to break down the door after worried relatives could not get an answer.
"Mr Philpott had been badly beaten and stabbed in what can only be described as a sickening and extremely cowardly attack.
"I do not believe this was a random killing and Mr Philpott may have known his killer or killers."
The victim had been with his identical twin brother on Thursday evening.
A team of forensic experts from Wetherby has been drafted in and are expected to be examining the flat for several days.
Police want to hear from anyone who saw Mr Philpott during the hours leading up to his death.