The FBI is investigating the 2013 death of a Virginia man who was repeatedly shocked with stun guns by police, the head of the federal agency's Richmond division said.

Adam Lee told The Associated Press that he personally went to the home of Linwood Lambert Jr.'s sister and told her the FBI is investigating. The federal inquiry is separate from a local prosecutor's investigation.

Virginia State Police previously looked into the death of Lambert while he was in the custody of officers in South Boston, a town of about 8,000 near the North Carolina border. Their findings were turned over to Halifax County Commonwealth's Attorney Tracy Quackenbush Martin, who has been criticized by the Lambert family and civil rights groups for not yet deciding whether the officers should be charged.

Martin, who previously told the AP she expected "a parallel investigation" by federal authorities, did not immediately respond to telephone and email inquiries about the status of her probe Thursday. She previously said she would take as long as necessary to make the right decision.

Lee said the FBI is "working in partnership" with South Boston and state police.

"This is not second-guessing, not looking over their shoulder," he said.

Civil rights leaders requested the federal inquiry because they believe questions remain unanswered, and they are unhappy with the pace of the prosecutor's investigation.

"A black man was killed while in custody of South Boston police," said Jack Gravely, executive director of the NAACP in Virginia. "Why did he die? What did he die of, and why has it taken the commonwealth's attorney more than two years to issue a report on the death of Linwood Lambert Jr.?"

David Heilberg, a defense attorney in Charlottesville who is not involved in Lambert's case, said he has never heard of an investigation of this kind taking so long.

Lee said that in addition to visiting Lambert's sister, Gwendolyn Smalls, he called the NAACP to report the FBI's involvement.

"We want to make sure the families are aware and interest groups are aware that if there's anything to it, it will be borne out by a thorough and complete review," Lee said.

Videos released in November show three officers using stun guns on Lambert multiple times after taking him to a hospital for a mental health evaluation. Lambert ran from the officers at the hospital, and instead of taking him to the emergency room, they took Lambert to jail.

An ambulance later brought him back to the same hospital, where he was pronounced dead.

An autopsy report says Lambert died of "acute cocaine intoxication." Attorneys representing Smalls in a $25 million lawsuit alleging the use of excessive force have argued that the videos cast doubt on that finding.

Linwood Lambert Sr. recently sent a letter urging Gov. Terry McAuliffe to press Martin to conclude her investigation. He wrote that "the manner in which my son died adds to the devastation" and has made it difficult for him and other family members to sleep and focus on day-to-day activities.

"It's been almost three years thus, we are seeking some form of closure," he wrote.