A service member opened fire on the Chattanooga gunman after he crashed the gates of a military reserve center last week, an FBI investigator disclosed on Wednesday.
The service member opened fire after the gunman, Mohammad Youssef Abdulazeez, got out of his car carrying an assault rifle, a handgun and ammunition, said the investigator, Ed Reinhold.
He said it was too soon to know whether the gunman was hit.
The gunman then entered the building and made his way through, shooting at people as he found them. He ran out the back of the building and killed two service members in a motor pool area, Reinhold said. Two service members "attempted to provide cover" and help people escape, he said.
"The heroic actions of these service members doubtlessly saved numerous lives," Reinhold said.
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Four Marines and a Navy petty officer were killed in the attack. The gunman was killed by Chattanooga police.
Reinhold said it was too early to say whether the gunman was radicalized. He said investigators are treating the gunman as a "homegrown violent extremist."
A Marine commander told reporters that Marines went room to room to shepherd their fellow service members to safety, and that some "willingly ran back into the fight."
"The legacy of that day is one of valor," said Marine Maj. Gen. Paul W. Brier.