In a statement, the American Civil Liberties Union said the investigations should look into the death of Ibragim Todashev, who was shot and killed May 22 during interrogation in Orlando, Fla., by FBI and law enforcement officials from Florida and Massachusetts.
Todashev was being interviewed about his relationship with Tamerlan Tsarnaev, a suspect in the Boston Marathon who was killed during a shootout with Massachusetts police.
"There are conflicting reports as to whether the Massachusetts troopers were in the room at the time of the shooting and whether their purpose at the interrogation was to investigate the Boston Marathon bombings, a 2011 triple-homicide in Waltham, or something else," the ACLU statement said. "The role of the Orlando police officers is even more unclear."
Howard Simon, executive director of the Florida ACLU, said officials there were deferring to the FBI, which has "offered completely incompatible explanations," CNN reported.
The ACLU charged the FBI had prevented the medical examiner from releasing the cause of death and pushed for the speedy deportation of a potential witness. The agency's final report will be classified, the group claimed.
Dennis Rodman pledges to end trips to North Korea
Interpol investigating stolen passports on missing flight