Muhammad Youssef Abdulazeez

Biden's comments came at a memorial to the five servicemen killed in a shooting rampage by Kuwait-born Muhammad Youssef Abdulazeez.

"These perverse ideologues, warped theocrats, they may be able to inspire a single lone wolf to commit a savage act, but they can never, never threaten who we are," Biden said. "When this perverted jihadist struck, everyone responded."

 His once clean-shaven face was covered with a bushy beard. A short time later, four Marines lay dead. A sailor who was seriously injured died Saturday.

He was believed to have been born in Kuwait, and it was unclear whether he was a U.S. or Kuwaiti citizen. The official was speaking on condition of anonymity to discuss the ongoing, sensitive investigation. It was not immediately clear whether the gunman's first name was spelled Muhammad or Mohammad.