The family and supporters of an unarmed Chechen, shot in mysterious circumstances by FBI agents investigating his friendship with Boston Marathon bombing suspect Tamerlan Tsarnaev, have accused US authorities of mounting a campaign of “intimidation and harassment” against his associates.

The girlfriend of Ibragim Todashev was deported at the weekend after spending two weeks at an immigration detention center in Florida. She had already spent several months in jail for having an expired visa earlier this summer. Another friend of Todashev is also in jail.

Todashev, 27, was shot on 22 May after being questioned about his friendship with Tsarnaev, one of two brothers suspected of carrying out the Boston Marathon bombing on 15 April that killed three spectators and injured more than 260. Several inquiries into Todashev’s death are under way.

His family believe that authorities investigating the Boston bombing have unfairly targeted people close to Todashev. Tatiana Gruzdeva, 20, who lived with Todashev at the apartment in Orlando, Florida, where he was killed, arrived in Moscow on Saturday morning after being deported from the US. Todashev’s father Abdulbaki and the Council on American-Islamic Relations (CAIR), a Florida-based Islamic advocacy group representing the family, told the Guardian that the deportation followed months of “hounding” of his son’s friends by federal authorities.

Another roommate, Ashurmamad Miraliev, 20, has been in jail since 18 September on a charge of tampering with a witness in a year-old assault case. The Florida chapter of CAIR said he was denied access to a lawyer. “People who had anything to do with him are being put behind bars. I don’t know why. It’s supposed to be America, it’s supposed to be a democracy,” Abdulbaki Todashev said.

FBI spokesman Paul Bresson said he had no knowledge of Gruzdeva’s deportation and would not comment on CAIR’s claim that Todashev’s friends had been intimidated.

But CAIR communications director Samantha Bowden said Gruzdeva, who is originally from Moldova, was told by FBI agents when she was detained earlier this month that a year’s extension to her visa, granted after her first period in jail, was being rescinded simply because of an interview she gave to a Boston magazine.

The Guardian has learned she was questioned briefly by Russian authorities on her arrival in Moscow, then released to join her mother. “We’re glad it was a successful deportation and that she is safe and well,” Bowden said.

Todashev, meanwhile, said he is still waiting for an explanation of his son’s death, almost five months after the shooting and the launch of separate inquiries by the FBI, the Department of Justice and Jeff Ashton, the state attorney for the Orlando area.

Speaking to the Guardian from his home in Grozny, he said that he has heard nothing, despite personal assurances from state and federal officials in Florida that he would be kept informed.

“They promised it would be an honest and just investigation, but so far there has been no information at all,” said Todashev, who met Ashton during a trip to the US in August. “I said to them: ‘Everything is obvious. You can investigate what you like, but everything is already clear.’”

Todashev was killed in May after FBI and other law enforcement agents questioned him for several hours about his friendship with Tsarnaev.

After the bombing, there was speculation that Tsarnaev may have been involved in a 2011 triple murder in Waltham, Massachusetts, in which his best friend was killed. According to initial media reports, Todashev had just confessed to having also been involved in the Waltham murder when he lunged at the FBI agents with a knife or ceremonial sword that was hanging on a wall.

However, several different accounts of the shooting soon emerged and the FBI backtracked from the initial assertion that Todashev was armed. Lawyer Hassan Shibly, executive director of CAIR’s Florida chapter, said an investigation by the group, including a private postmortem, concluded that Todashev was shot seven times, including once in the back of the head.

“We have also been able to ascertain that only one agent drew his weapon, and fired the shots,” Shibly said. “Agents are trained to draw their weapons the minute there is a threat. If Ibragim was really such a threat, why did the other officers not draw their weapons?”

Abdulbaki Todashev said he was convinced his son was subjected to a cold-blooded execution by federal agents acting “worse than bandits”. He said his son knew Tsarnaev only casually from a mixed martial arts gym in Boston they both used to attend and that he was almost immobile from recent knee surgery and unable to threaten anybody.

Apart from brief statements in which the FBI claimed Todashev turned violent during questioning, the agency has consistently refused to comment. Bresson said: “The review is still ongoing. Keep in mind it’s not just the FBI conducting a review. The Department of Justice and other outside entities are also reviewing. While that is still ongoing, we cannot discuss.”

CAIR says that Gruzdeva and Miraliev were both held without access to lawyers. The pair acted as drivers for Todashev during his recent visit to the US. “It’s intimidation and retaliation, pure and simple,” Bowden said. “These are two people very close to Ibragim, who helped his father in his quest for justice while here, who have spoken out against the FBI for its overreach and intimidation tactics.

“Neither had any connection to any terrorism whatsoever, yet the FBI has been following their every move and denied them the right to an attorney and of free speech.”

The progress of the Department of Justice inquiry is uncertain, and its press office is currently closed due to the US government shutdown. A spokeswoman for state attorney Ashton told the Guardian only that his investigation was ongoing and that he would not comment until it was complete.

Bowden said that CAIR would wait for the results of the inquiries before deciding on possible civil legal action, but that the group was “sceptical of the FBI policing itself”.

Tamerlan Tsarnaev, 26, was killed in a shootout with Boston police on April 19, four days after the bombing. His brother Dzhokhar, 20, remains in custody charged with murder and using a weapon of mass destruction.