President Barack Obama promised a full investigation into shootings at military sites in Tennessee that killed four Marines, described as a potential act of terrorism by U.S. officials.

"We take all shootings very seriously, but obviously when we have an attack on a U.S. military facility, we have to make sure we have all the information necessary to make an assessment of how this attack took place," Obama said in the Oval Office after being briefed on the shootings by FBI Director James Comey.

The shootings, under investigation by the Federal Bureau of Investigation and local law enforcement, took place just miles and about 30 minutes apart at a U.S. Navy Network Operations Support Center and an armed forces recruiting center in Chattanooga. Names of those slain were not released.