WASHINGTON, Oct. 30 -- A notorious US mob boss who was convicted in 2013 of multiple murders and other crimes was found dead at a federal prison in state of West Virginia, authorities said Tuesday.

James "Whitey" Bulger, 89, was pronounced dead by a medical examiner after attempts to revive him failed, the West Virginia prison said in a statement.

Bulger was found unresponsive at approximately 8:20 am Tuesday, according to the prison, where he was transferred a day ago.

Details on how Bulger died were not immediately known.

Local news outlets said his death was being investigated as a homicide, citing unidentified sources.

Bulger was sentenced to life in prison after being convicted of participating in 11 murders, extortion, money laundering and weapons charges in a high-profile trial five years ago.

He was arrested in Santa Monica, state of California, in 2011 after 16 years at large and 12 years on the FBI's most-wanted persons list.

Bulger ran his operation out of the northeast city of Boston, state of Massachusetts.