The father of a Chechen man shot dead while being interviewed by the FBI in Florida last week over his connection to one of the Boston bombing suspects has accused US agents of killing his son "execution-style".

Speaking in Moscow, Ibragim Todashev's father, Abdul-Baki Todashev, said his son was "100% unarmed" but had been shot six times in the torso and once to the back of the head. He showed reporters photographs of the body which he said he had been taken in a Florida morgue by a friend, Khusen Taramov.

"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," Todashev said. "These just aren't FBI agents – they're bandits," he added.

The dead man's family will submit a formal request on Friday for a federal inquiry into the shooting, the Guardian has learned.

An Islamic advocacy group insists that Todashev was unarmed, and that he was hit by at least seven bullets during the interview in his Orlando apartment on 22 May, contradicting earlier law enforcement accounts of the incident.

Hassan Shibley, executive director of the Florida chapter of the Council on American-Islamic Relations, said that Todashev's family and friends were meeting lawyers on Thursday to finish off a report that will support the request.

"We're not accusing anybody of anything, but we do want to know how an unarmed man who had not been charged or convicted of anything was shot seven times, once in the head, and killed," he said.

He said his group had a source close to the FBI investigation who confirmed that Todashev was not armed, and that all but one of the agents had left the room when the fatal shots were fired.

"We had heard from day one from his family that he didn't have a weapon," Shibley said. "We just want to find out what happened and make sure that Mr Todashev's civil rights were not violated."

The FBI, which initially suggested that Todashev, 27, had "flipped out" and initiated a "violent confrontation", during which he lunged at agents with a knife, would not comment to on the new allegations, although unnamed sources close to the inquiry told the Washington Post that it now seemed apparent he was not armed.

FBI spokesman Paul Bresson said in a statement that an agency taskforce from Washington was already in Orlando reviewing the case.

"The FBI takes very seriously any shooting incidents involv­ing our agents and as such we have an effective, time-tested process for addressing them internally," he said in a statement.

"The review process is thorough and objective and conducted as expeditiously as possible under the circumstances."

He said that the panel, which included Justice Department investigators and the FBI's shooting incident review group, "examines all of the information and determines the reasonableness of the application of deadly force in accordance with the Department of Justice's deadly force policy and the law."

At the time of his death, Todashev was under investigation over his friendship with Tamerlan Tsarnaev, one of the two Chechen men suspected of the April 15 bombing attack on the Boston marathon that killed three people and injured more than 260.

He and Tsarnaev were also suspects in a triple murder in Waltham, Massachusetts, in 2011, and he was reported to have been about to sign a confession for those killings when he was shot.

Todashev's family, however, say the pair only knew each other casually through a shared interest in martial arts when they both lived in Boston, and that he had always denied any connection with the bombing during numerous FBI interviews in the weeks following.

His widow, Reniya Manukyan, appeared at a press conference in Orlando on Wednesday, hosted by CAIR, to support the call for a federal review of the case. "We want to know why it happened; we want to know what is the truth," she said.

Manukyan, who lives in Georgia, said she had been married to Todashev for three years. She claimed she had documents that proved they were together on September 11, 2011, the day of the Waltham murders in which a close friend of Tsarnaev was killed. The three victims were found with their throats slit and their bodies covered with marijuana and banknotes.

At the press conference, Shibley also showed reporters photographs of Todashev's body, taken by Taramov.

"They were talking to both of us. And they said they need him for a little more, for a couple more hours, and I left, and they told me they're going to bring him back. They never brought him back," he said after the shooting.

Tamerlan Tsarnaev was killed in a shootout with Boston police four days after the bombing. His brother Dzhokhar, 19, was captured a day later and remains in custody on federal murder charges. He faces the death penalty if convicted.