Terror comes to the US, 4 Marines killed after gunman attacks 2 military sites in Tennessee

The suspected shooter is also dead, Chattanooga Mayor Andy Berke said. Investigators "have not determined whether it was an act of terrorism or whether it was a criminal act," Ed Reinhold, FBI special agent in charge, told reporters.

"We are looking at every possible avenue, whether it was terrorism -- whether it was domestic, international -- or whether it was a simple, criminal act."

The shootings unfolded at two sites over 30 minutes, Reinhold said. The suspected gunman started spraying bullets at the glass doors of a strip mall military recruiting center around 10:45 a.m., witnesses said.

From there, he headed to another location more than seven miles away, an operational support center operated by the U.S. Navy.

That's where the four victims and the gunman were killed, Reinhold said.

At least three people were injured, officials said, including a police officer and a military service member.

The officer, who was injured when he was shot in the ankle, has been treated at the hospital, Berke said. A military service member was also among the wounded, the Defense Department said.

 on Thursday, the U.S. Navy said.