(CNN) He'd shot at a military recruitment center, drove off, and rammed the gates of a Navy reserve center. A service member inside the facility saw the threat posed by the heavily armed Mohammad Abdulazeez and opened fire.

But tragically, that didn't stop Abdulazeez.

Rather, Abdulazeez forced his way into the Navy building in Chattanooga, Tennessee, and killed one service member, FBI Special Agent in Charge Ed Reinhold said Wednesday. He continued his rampage, exiting through the back, and fatally shooting four more. Then he tried to get inside again.

He didn't make it in this time -- he was shot by one of the Chattanooga police officers who confronted him.

The whole episode lasted no more than five minutes. Long enough for four Marines and one Navy sailor to be fatally wounded -- as was Abdulazeez.

"We are incredibly thankful for the bravery of the responding police officers," said U.S. Marine Maj. Gen. Paul W. Brier. "Their immediate actions that day saved lives."

They weren't the only ones who acted heroically last Thursday, July 16, according to Brier. He also lauded the service members -- all but two, apparently, weren't armed -- who risked their lives to help others and stop the carnage.

"I can tell you that our Marines reacted the way you would expect," the major general said. "Rapidly going from room to room, they got their fellow Marines to safety.

"Once they had gotten to safety, some willingly ran back into the fight."

FBI: Shooter acted alone, motive still unknown

Exactly why they were thrust into this fight -- not in a warzone in a place like Iraq or Afghanistan, but in otherwise placid southeastern Tennessee -- is a question investigators are trying to answer.

What they do know is that Abdulazeez is responsible and, on that day at least, he acted alone.

Born in Kuwait and with Jordanian citizenship, Abdulazeez was a naturalized U.S. citizen who was a once-devoted, disciplined mixed-martial-arts fighter; a top student known for his smarts, charm and humor; and a devout Muslim who kept in touch with his roots in the Middle East.

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Yet friend Abdulrazzak Pirzada said that Abdulazeez changed after spending time in the Middle East. Jordanian sources said Abdulazeez had been in Jordan as recently as 2014 visiting an uncle. He visited Kuwait and Jordan in 2010, Kuwait's Interior Ministry said.

"Something happened over there," Abdulrazzak Pirzada said of Muhammad Youssuf Abdulazeez, adding that "he never became close to me like he was before he went overseas. ... I'm sure he had something that happened to him."

Reinhold, from the FBI, acknowledged numerous questions surrounding "the shooter and why he did what he did."

"We have those same questions," he said, "and are trying to get those answered."

Carrying an assault rifle, handgun, ammo

While authorities didn't offer much Wednesday on Abdulazeez's motive, they did shed new details on what he did and how others -- military service members and police alike -- responded.

He started at a military recruitment center on Lee Hwy in Chattanooga. From inside a rented silver Mustang, Abdulazeez opened fire and wounded one recruiter there before speeding off.

Chattanooga police were soon on his tail, but not on his bumper, Reinhold said. Thus, they weren't able to stop him before he drove about seven miles and through the gates of the Navy operations support center on Amnicola Hwy.

Abdulazeez emerged from the car with "an assault rifle, a handgun and numerous magazines of ammunition," according to the FBI agent.

"A service member from inside the facility observed him and opened fire on him, firing several rounds," Reinhold said. "The shooter then responded by shooting into the building."

Authorities did not say whether or not the service member was authorized to carry a weapon at the facility, with Reinhold saying that's something the military could look into. The FBI agent added that investigators found a total of two firearms that belonged to service members, though it wasn't clear if the second one was discharged; he did not say whether the firearms were personal or military weapons.

Once inside, Abdulazeez managed to kill one service member. (It was not immediately clear if that person killed was the same one who'd fired at him.) He then made his way to a gated motor-pool area, where he fatally shot two others, according to Reinhold.

Abdulazeez killed two more service members before apparently trying to head back inside. Early information suggests that one gun was used to kill all five military personnel, ruling out the possibility of friendly fire.

"At (that) point, he was engaged by the Chattanooga Police Department," Reinhold said. Police "returned fire and neutralized the shooter."

By doing so, the FBI agent added, the police "doubtlessly saved numerous lives."