The mugshot of Mohammad Youssuf Abdulazeez (booked as Mohammad Youssduf Abdulazeer, according to Hamilton County police documents) after he was arrested on April 20, 2015 for a DUI. Police handout

Attorney General Loretta Lynch said in a statement that she had "directed the FBI to take the lead in the national security investigation of this heinous attack on members of our military."

Soon after the shootings, Mayor Andy Berke said “This is a nightmare for the city of Chattanooga."

Within hours of the bloodshed, law officers with guns drawn swarmed what was believed to be Abdulazeez's house, and two women were led away in handcuffs. It was not clear who they were, and on Thursday night, Berke said he could not comment whether or not they had been arrested.

A dozen law enforcement vehicles, including a bomb-squad truck and an open-sided Army green truck carrying armed men, rolled into the Hixson neighborhood, and police closed off streets and turned away people trying to reach their homes

The shootings happened minutes apart, with the gunman spraying dozens of bullets at a recruiting center for all branches of the military and then apparently driving to a Navy-Marine training center 7 miles away, authorities and witnesses said. The attacks were over within a half-hour.

A military official said the Marines were slain when shots were fired in a Navy-Marine center. The wounded Marine was hit by gunfire fired at an Armed Forces recruiting center not far away, the official said on condition of anonymity.

“Lives have been lost from some faithful people who have been serving our country, and I think I join all Tennesseans in being both sickened and saddened by this,” Gov. Bill Haslam said.

Sgt. 1st Class Robert Dodge, 36, the center leader for U.S. Army recruiting at the facility on Old Lee Highway, said four Army personnel were in the office at the time. He said the Air Force, Navy, Marine Corps and National Guard all have their own offices right next to each other. Around 10:30 or 10:45 a.m., Dodge and the others heard a gunshot, “which kind of sparked our attention,” he said.

“Shortly after that, just a few seconds, the shooter began shooting more rounds. We realized it was an actual shooting,” he said. They then got on the ground and barricaded themselves in a safe place. Dodge estimated there were 30 to 50 shots fired. He did not see the shooter or a vehicle.

Marilyn Hutcheson, who works at Binswanger Glass just across the street from the U.S. Naval Reserve building on Amnicola Highway, said she heard a barrage of gunfire around 11 a.m.

“I couldn't even begin to tell you how many,” she said. “It was rapid fire, like pow pow pow pow pow, so quickly. The next thing I knew, there were police cars coming from every direction.”

President Barack Obama, who received a briefing Thursday afternoon from FBI director James Comey, spoke from the Oval Office. He called the attack a "heartbreaking circumstance for these individuals, who served with great valor, to be killed in this fashion."

He stopped short of using the word "terrorism," adding that "we have to make sure we have all the info necessary to make an assessment of how this attack took place."

Abdulazeez graduated from the University of Tennessee at Chattanooga in 2012 with a bachelor's in electrical engineering, according to university spokesman Mike Andrews and the Tennessee Valley Authority, the federally owned utility that operates power plants and dams across the South, confirmed that he was a student intern there a few years ago..

He was arrested on April 20, 2015 on a DUI charge, Hamilton County police records show.