The father of a Chechen man shot dead while being questioned over the Boston Marathon bombing has written to US president Barack Obama asking for help to find out what happened.
Abdulbaki Todashev wrote to Mr Obama saying his son Ibragim Todashev knew dead bombing suspect Tamerlan Tsarnaev and his brother Dzhokhar through boxing, but had no connection to the outrage or any other crime.
He said his son went voluntarily to an FBI office in Orlando, Florida, to speak to agents four times before they arrived at his apartment on May 22. He included photos of his son's body with the letter and offered to travel to Washington DC to speak to the president directly.
"My reaching out to you is dictated by the calling of my soul and the unsubsiding pain of the father who has lost his guiltless son to a violent shooting death," Mr Todashev wrote.
"I am not asking you to share this pain with me; rather, I plead with you, the ruler of the great country and a guarantor of the democracy in the modern world, to see that the law and justice triumph in my son's case."
National Security Council spokeswoman Caitlin Hayden said the White House was reviewing the letter "to determine the appropriate follow-up". She referred questions to the FBI, which declined to comment.
Lawyer Barry Cohen said Mr Todashev, who lives in Russia, feared a cover-up. "He's asking the president from one dad to another not to let that happen," he said.
Mr Todashev wants to know why authorities are being secretive about the investigation and why it is taking so long, Mr Cohen said.
An Orlando prosecutor investigating the death said he expects to release a report next year. Government authorities are also investigating.
Ibragim Todashev, a mixed martial arts fighter recovering from knee surgery, was killed as FBI agents and police from Massachusetts and Florida questioned him. Officials originally said he lunged at an agent with a knife, but later said it was no longer clear what happened.
A court filing this year indicated Ibragim Todashev had told investigators that Tamerlan Tsarnaev participated in an unsolved 2011 triple murder in Waltham, just west of Boston. It was unclear if he provided any evidence of that.
Dzhokhar Tsarnaev has pleaded not guilty to the April 15 marathon bombing, which killed three people and injured more than 260 others. His brother died following a shootout with police.