The government's independent advisor on the activities of the security services has met the family of Kieran Doherty in Londonderry.
Lord Carlile is to examine the family's allegations that MI5 was involved in Mr Doherty's death.
The Derry man was a member of the Real IRA and was murdered by the organisation in February.
Lord Carlile said he will listen to the Doherty family's concerns and will speak to the security services.
He will report his findings to the Secretary of State.
Lord Carlile met the family at the spot where Mr Doherty's body was found.
Kieran Doherty's uncle, Vincent Coyle, said the family took comfort from Lord Carlile's visit.
"It's a huge step forward," he said, "And a mark of the man, Lord Carlile, a man of great integrity, that he has stepped forward in this way.
"He will do what he can. The problem for us of course is that we would prefer an ombudsman, someone with 'larger teeth', but I have no doubt Lord Carlile will carry out the report and put it in front of the Secretary of State, and then go higher again."
The Liberal Democrat peer previously said his experience in the legal system will help him find out the truth about the killing.
"I have been around as a lawyer, as a part-time judge, and as independent reviewer of terrorism legislation for long enough to be able to sniff out any attempt at concealment, I hope. If I find there has been concealment, I will make public that belief," he said.
Kieran Doherty was found shot dead outside Derry in February.
The Real IRA claimed responsibility for the murder and said the 31-year-old had been killed for his involvement in the drugs trade.
However, his family rejected the claim and said they believed members of the security services may have been involved in his murder.
Kieran Doherty was a senior member of the Real IRA and a lifelong republican.
In the months before he was killed, he told a local newspaper that MI5 had tried to recruit him as an agent.
The offer to meet Kieran Doherty's family was made in a letter to Foyle MP Mark Durkan, who has lobbied on their behalf. He welcomed the move, but urged caution about the outcome of the investigation.