The father of a Chechen man shot and killed by FBI agents investigating the Boston Marathon bombing has hired two prominent Florida civil rights attorneys in his attempt to uncover the truth about his son's death.

Abdulbaki Todashev spoke at a Tuesday press conference in Tampa at which the Council on American-Islamic Relations (CAIR), a Muslim advocacy group, announced that lawyers Barry Cohen and Eric Ludin had been retained to represent the dead man's family as three separate investigations into the 22 May shooting progressed.

Federal agents killed Ibragim Todashev, 27, an acquaintance of Boston bombing suspect Tamerlan Tsarnaev, in his Orlando apartment after several hours of interrogation. The FBI has never discussed details of the incident, but launched an internal inquiry, while Jeff Ashton, state attorney for Orange and Osceola counties, announced his own criminal investigation last week.

CAIR officials, meanwhile, have hired experts to conduct their own examination of the evidence, and said Tuesday they reserved the right to launch civil action depending on the results of the various inquiries.

"It's not just about the Muslim community, it's about the civil rights of all Americans," Hassan Shibly, the group's executive director, said.

"No legal authority is above the law. This case is about police accountability and justice. I don't think we want to live in a country where people get shot by federal agents in their own homes."

Todashev, speaking through a translator, paid tribute to his son as "a good boy who didn't do anything wrong".

He said he was "certain we will be able to achieve justice" and added that his unarmed son's life was cut short just days before he was due to visit family in Russia, having given his full co-operation to investigators.

"He was simply not capable of doing it," Todashev said, referring to claim in an early FBI statement that Ibragim Todashev, who had trained at the same mixed martial arts gym in Boston as Tsarnaev, lunged at the agents. His son had recently undergone knee surgery and was incapable of leaping at anybody, he said.

"He was responsible and conscientious. He was a good brother, a good family member, we all loved him very much and he loved us back."

Todashev's quiet demeanour was markedly different from a fiery press conference he gave in Russia in May, at which he denounced the FBI agents as "bandits" who murdered his son "execution-style".

"I'd only seen and heard things like that in the movies – they shoot somebody and then a shot in the head to make sure," he said then, commenting on CAIR's assertion, based on autopsy photos and reports of friends who prepared the body for burial, that the younger Todashev was shot six times in the torso and once in the back of the head.

Cohen, a Tampa-based attorney who was not at the press conference, is no stranger to high-profile cases and is renowned as a media-savvy lawyer who specialises in representing clients against federal agencies or corporate giants.

Ludin, a senior partner of the Clearwater firm Tucker Ludin, was keen to keep media questioning away from speculation about the shooting incident and what he said was "just a rumour" that Todashev was being questioned as a suspect in a 2011 triple homicide in Waltham, Massachusetts, in which a close friend of Tsarnaev was a victim.

"We have theories of what happened but it would be inappropriate to theorise and it could interfere in some way with Mr Ashton's investigation," Ludin said.

"We want to be as co-operative as possible and so we aren't going to comment on what we think happened in that room except that we think it was not a justified killing."

But he did say he "had no reason there was any kind of deadly weapon in the possession of our client's son".

He said he had no idea how long Ashton's inquiry would take but that he had urged the older Todashev, who was scheduled to return to Russia this week, to stay in the country at least until he had the chance to meet with Ashton.

The FBI, he added, had tried to talk to Todashev, but that he had declined an interview without a lawyer present.

Ludin said he understood why the FBI had requested the coroner's office to withhold the autopsy report because it was relevant to an inquiry concerning possible criminal behaviour by law enforcement officers, and its release could compromise the investigation.

"My client's son may have been an acquaintance with a person involved in an evil act, but merely being an acquaintance, merely being a Muslim, merely being from a Russian province, does not mean he isn't deserving of proper treatment by law enforcement officers," he said.

The FBI did not respond to a request from the Guardian for comment, but a spokesman said recently that the agency is conducting a "thorough" inquiry into the incident.