A federal prosecutor says he will not pursue federal criminal civil rights charges against three Virginia police officers who used stun guns multiple times on a black man before his death.

U.S. Attorney John Fishwick Jr. said Thursday that an investigation into death of Linwood Lambert Jr. found insufficient evidence to prove that the three officers violated federal law by willfully using unreasonable force against the man.

Lambert died after being taken into custody by South Boston Police Officers Tiffany Bratton, Clifton Mann and Travis Clay in May 2013.

Police dash cam and hospital surveillance videos released last year show officers using stun guns on Lambert multiple times after taking him to a hospital for a mental health evaluation.

An autopsy report said Lambert died of "cocaine-induced excited delirium."