Detective Superintendent Mel Shore of West Mercia Police said police were particularly keen to trace two people who were seen visiting Errol McGowan's home at 1030 BST on the morning of his death.
Relatives of doorman Harold "Errol" McGowan, 34, and his nephew Jason, 20, joined detectives at a police press conference on Friday.
Detective Superintendent Mel Shore of West Mercia Police said police were particularly keen to trace two people who were seen visiting Errol McGowan's home at 1030 BST on the morning of his death.
"They are crucial to our investigation," he said.
The McGowan family believes the two deaths in Telford, Shropshire, were racist murders.
A murder investigation is under way after the deaths were initially thought to be suicide by police.
Errol McGowan was found dead on 2 July last year. His nephew was discovered hanging from railings on New Years Day.
Noel McGowan said his brother Errol had told family members in the days leading up to his death that he feared for his own safety and that of his partner Sharon Buttery.
'Heartfelt plea'
This was because he had been subjected to racist harassment, Mr McGowan told the press conference.
He said he was making a "heartfelt plea" for anyone with information to go to the police in confidence.
Leroy McGowan, also Errol's brother, added that if people were afraid to approach the police they should tell someone in the community.
"Tell your family, churches or ring Crimestoppers," he said. "The reason we want the community to co-operate with the police is so we can find out the truth about Errol and Jason."
Jason McGowan's widow Sinead said: "Please don't be a bystander, people who commit these kind of crimes will boast about it and if they have told you anything, please get in touch."
Sharon Buttery made a special plea to wives, girlfriends or mothers of anyone involved to come forward.
Alleged mishandling
The family were joined by the head of the Metropolitan Police Racial and Violent Crime Task Force John Grieve, brought in after the McGowan family complained that the West Mercia force assumed the hangings were suicides.
Mr Grieve appealed for help from local people. "The police can't do it on their own," he said.
The dead men's family met earlier this year with Home Secretary Jack Straw to discuss alleged police mishandling of the investigation.
They were told in February that Mr Grieve would provide "support and advice" to West Mercia officers, who have failed to find any evidence of foul play.
But not all family members believe the deaths were suspicious. Robert King, the father of Jason McGowan, has said his son took his own life and urged an end to "lies and rumours" that he was killed by racists.