The father of a Chechen man who was fatally shot by a Boston FBI agent has arrived in the United States, where he intends to file a lawsuit against the agency and investigate the mysterious death of his son.

Abdulbaki Todashev is the father of 27-year-old Ibragim Todashev, a mixed martial arts fighter who was fatally shot seven times by the FBI during an extensive interview about his connection to alleged Boston bomber Tamerlan Tsarnaev, as well as any knowledge about a triple murder in Waltham, Massachusetts. Todashev, who lived in Orlando, Florida at the time of the interrogation, had been friends with the suspected bomber when they both lived in Massachusetts.

After several hours of questioning, Todashev was shot dead by the FBI in a case that continues to be shrouded in mystery. The man’s father, who lives in Chechnya, described the incident as an execution-style murder.

“He was shot seven times,” Mr. Todashev told TIME. “In the heart and in the head. What is that if not murder?”

Conflicting reports surfaced after the man’s death, with some suggesting that the young Todashev attacked an FBI agent with a knife, metal pole, or broomstick. Other reports claimed the Chechen-American was unarmed.

“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,” Howard Simon, the Florida executive director of the American Civil Liberties Union (ACLU), told TIME.

The victim’s grieving father does not believe that his son posed any serious danger to the officers who were interrogating him, especially since he was injured. A photograph of his son’s dead body shows a row of stitches over the right knee.

“He had just had surgery on his knee and was still walking with crutches,” Todashev said.

The FBI has opened an internal investigation into the killing, but refuses to comment on the case and blocked the release of the victim’s autopsy report.

The distressed father says he is determined to independently discover the truth behind his son’s death, and plans to travel to the US, where he will employ the help of human rights organizations to get answers. Todashev speaks little English, cannot afford a lawyer, and has nothing more than a tourist visa to the US, but the ACLU has vowed to help him.

He says he wants to sue the FBI - not for compensation, but for answers about his son’s death. However, uncovering the truth is likely to come with numerous hurdles: state investigators in Florida last week rejected an ACLU request for an independent investigation into the shooting, claiming it would be “inappropriate” for the human rights organization to intervene.

“Secrecy fosters suspicion and the people of Florida deserve better than to be left without an explanation from their government about what led to a person being shot to death,” Simon said in a statement last Wednesday. “This means it is more likely that the family of Ibragim Todashev will have to file a lawsuit in order to get answers about how their son died.”

An ACLU request to investigate the death was also rejected by the Massachusetts attorney general.

Todashev announced his plans to sue the agency just a few days after the ACLU’s requests were denied, and told TIME that he would file a wrongful death suit - which would force the FBI to reveal all the facts of his son’s death and accept responsibility for any wrongdoing.

“At least once they must be made to admit they were wrong,” he said. “What, is the FBI infallible?”