Maxim Shemetov / REUTERS Abdulbaki Todashev, the father of Ibragim Todashev, attends a news conference in Moscow on May 30, 2013

A grudge against the FBI is never an easy thing to act upon, especially for a man as foreign to the U.S. legal system as Abdulbaki Todashev, a municipal official from the Russian region of Chechnya. But on Monday, Aug. 5, Todashev arrived in Tampa, in Florida, with a black briefcase of photographs — the evidence he plans to use in suing the FBI for the wrongful death of his son. The case would be a long shot, in part because Todashev speaks little English, cannot afford a lawyer and only has a U.S. tourist visa glued into his Russian passport. What he does have is the help of two U.S. rights organizations — including the American Civil Liberties Union, or ACLU — and the determination of a grieving father from a region where blood feuds run deep.

Todashev’s eldest son, Ibragim, was killed during an FBI interrogation in his home in Orlando on May 22, two days before he was due to fly home to his native Chechnya. The FBI, along with several officers from the Orlando and Boston police forces, had arrived at his one-bedroom apartment that evening to interrogate him in connection with the Boston Marathon bombing. One of the bombing suspects, Tamerlan Tsarnaev, who was also an ethnic Chechen, had been a friend of the younger Todashev when they both lived in Massachusetts. The FBI was trying to learn more about their relationship, so the officers questioned him for several hours that night at a table in his living room. But soon after midnight, under circumstances that remain unexplained, Ibragim was fatally shot.

The photographs in his father’s briefcase seem to raise more questions about the death than they answer. On a recent afternoon in Moscow, Todashev laid them out across the table of a diner, starting with the family photos he had taken of his son with his 11 siblings in Chechnya. In one of the frames, Ibragim stands with several of his younger brothers at a boxing club in Grozny, the regional capital, where he began his training to become a mixed-martial-arts fighter. In another, he grapples during a professional cage fight in Florida, surrounded by rows of American fight fans. Then his father shows the photos of his body, rent with wounds, that his friends in Florida had taken while preparing him for burial. One closeup of the top of his head appears to show two bullet holes about half an inch apart from each other. “He was shot seven times,” his father says. “In the heart and in the head. What is that if not murder?”

The FBI, which has opened an internal investigation into the killing, has done little to explain how it went down. In a statement on the day of Ibragim’s death, the FBI’s Boston division said he had “initiated a violent confrontation” with the officers who had been questioning him. A week later, the FBI said in another statement that the death would be investigated by the Shooting Incident Review Group, which includes officials from the FBI and the Department of Justice. While that probe is ongoing, the bureau said, it cannot comment on the details of the case. On July 16, a Florida medical examiner said the FBI had blocked the release of Ibragim’s autopsy report pending the agency’s investigation.

U.S. media reports have meanwhile painted a confused and sometimes contradictory narrative. The most detailed account came from John Miller, a former deputy director of the FBI who now works as a correspondent for CBS News. On May 31, he said that right before Ibragim was killed, he had been writing out a confession to a triple homicide allegedly committed with Tsarnaev in Waltham, Mass., in 2011. Citing unnamed sources, Miller said that Ibragim then overturned the table, knocking the FBI agent back and charging at him with “a metal broom handle or some object like that.” Other media reports, also citing unnamed officials, have said Ibragim was wielding a knife or was unarmed.

The photos that his father showed TIME in Moscow seem to corroborate at least part of Miller’s account. In June, the elder Todashev received a U.S. tourist visa and traveled to Florida to make inquiries about his son’s death. There he met with the Council on American-Islamic Relations (CAIR), an advocacy group based in Washington, D.C., and with their help, he hired a private detective to question his son’s neighbors and access his apartment.

In the living room where the interrogation took place, the elder Todashev took several photographs, one of which shows a table lying on its side. “Apparently he knocked it over,” Todashev says, referring to his son. “You can see the things that were on the table are scattered on the floor.” A large pool of blood can be seen on the other side of the room, in the doorway leading to the kitchen. Citing a police document he says he saw in Florida listing the items seized from his son’s apartment, Todashev also said the FBI had confiscated “some kind of stick,” possibly a broom handle or the leg of a chair, along with a computer and other possessions.

But he denies that his son could have posed any serious threat to the officers interrogating him. “He had just had surgery on his knee and was still walking with crutches,” Todashev says, pointing to a photograph of his son’s dead body — over the right knee there is a long, neat row of stitches. He also denies that his son could have been involved in the triple homicide in Waltham or in the Boston Marathon bombings. Had Ibragim felt at risk of prosecution, his father reasons, he would have fled to Chechnya. Instead, he underwent numerous interrogations in Florida, and on the advice of the FBI, he canceled a trip he had planned to Chechnya in May. “I told him to come home,” his father says. “But he said he’d better stay, because the FBI asked him to.”

Ibragim, who was 27 when he died, first went to the U.S. in 2008 to study English as part of a student-exchange program. Earlier this year, he received a green card, making him a legal permanent resident of the U.S., where he married an Armenian American, Reni Manukyan. Although the couple was estranged, Manukyan has also been campaigning for justice in Ibragim’s death. The family has received some moral support from authorities in Russia, but little else. Ramzan Kadyrov, the Kremlin-backed leader of Chechnya, has claimed that Ibragim was killed “for no reason,” possibly as a “reprisal” by U.S. special services. But Ibragim’s father, a senior official in the city government of Grozny, told TIME that he had not discussed his son’s case with Kadyrov and was not receiving any assistance from the Russian state.

For now, Todashev says his best hope for clarity in his son’s death lies with the ACLU, which called for an independent investigation into the case on July 22. “The FBI has offered completely incompatible explanations, they have failed to explain how these inconsistent stories found their way into newspaper accounts of the shootings, and have not offered any clarifying comment about what really happened, ” said Howard Simon, the Florida executive director of the ACLU.

That statement also cited a June 18 report from the New York Times, which found that from 1993 to 2011, FBI agents shot about 70 “subjects” to death and wounded about 80 others; in every one of those cases, the agency’s internal investigations found its agents’ actions to be “justified.” That report has raised “public skepticism in the FBI’s ability to investigate itself,” said the ACLU, which sent requests to the Massachusetts attorney general and the Florida Department of Law Enforcement asking them to investigate Ibragim’s death independently.

Both those requests have been denied, leaving a wrongful-death suit against the FBI as the Todashev family’s only legal recourse. “They really don’t leave any other option than for the Todashevs and his survivors to go ahead with their civil claim,” says Yvette Acosta MacMillan, a staff attorney for the ACLU in Florida. “They would be able to obtain the records and the information through discovery in a lawsuit because right now, none of the information, none of the documents are being released.”

Two days before departing for Florida, where Todashev was due to meet with CAIR and the ACLU, he told TIME that he planned to file a wrongful-death suit, not to seek financial compensation, but to force the FBI to reveal the facts of his son’s death and, if they are damning, to accept responsibility. “At least once they must be made to admit they were wrong,” he said. “What, is the FBI infallible?”