CHATTANOOGA: The death toll in the mass shooting of US military personnel in Tennessee rose to five on Saturday as investigators pursued the motive behind the rampage. In a brief statement, the US Navy said a petty officer succumbed overnight to wounds he sustained in Thursday's attack on two military centres in Chattanooga. The Navy did not identify the sailor, but relatives named him as Petty Officer Randall Smith, a father of three daughters who had recently re-enlisted and transferred to Chattanooga. "It's hard to understand how somebody can hurt somebody that's serving for you, for your freedom, for your safety," his step-grandmother Darlene Proxmire told WANE television in Indiana. Four Marines also died in the attack — which authorities are treating as "an act of terrorism" — before the gunman, Mohammad Youssuf Abdulazeez died in a shootout with police. The FBI has asked foreign intelligence agencies to help trace Abdulazeez's movements and activities abroad.