Four U.S. Marines were killed on Thursday at two military bases by a 24-year-old gunman in Tennessee.
The Federal Bureau of Investigation (FBI) identified the shooter as Mohammod Youssuf Abdulazeez, who was also killed in a firefight with the police.
Three other persons were injured, including a police officer, a Marine Corps recruiter and a Navy sailor.
The last such incident took place in 2013, when an attack on a Navy Yard in Washington left 12 dead. In 2009, an attack on Fort Hood in Texas left 13 dead.
United States Attorney General Loretta Lynch was quoted, as saying that the FBI would take the lead in the "national security investigation of this heinous attack on members of our military.
Abdulazeez, a Muslim, lived in a leafy suburb of Chattanooga. US media reported that he was a naturalized American citizen born in Kuwait and had graduated from the University of Tennessee at Chattanooga with a degree in engineering.
Defense Secretary Ashton Carter and Navy Secretary Ray Mabus have offered their condolences to the families of the victims, with Mabus calling the attacks "both devastating and senseless.