The US Federal Bureau of Investigation on Thursday identified the gunman in two shootings at military centers in Tennessee that left four Marines dead as 24-year-old Mohammod Youssuf Abdulazeez.

"While it would be premature to speculate on the motives of the shooter at this time, we will conduct a thorough investigation of this tragedy," the FBI said in a statement, adding that three people were injured in the rampage.