CHATTANOOGA - The motivation for Muhammad Youssef Abdulazeez's murderous rampage remained a mystery to investigators Thursday night, some 12 hours after the separate shooting incidents that left four Marines dead and several others injured.

"We have no idea at this point what his motivation was behind this shooting," said Ed Reinhold, FBI Special Agent in Charge. "At this point we don't have anything that ties in directly to an international terrorist organization. I can't speculate on what his motivation may have been or what affiliation he may have had."

Reinhold spoke to the media at 11 p.m. local time Thursday in an auditorium at the TVA complex. He was joined by U.S. Attorney William Killian, Chattanooga Mayor Andy Berke, Tennessee Gov. William Haslem and U.S. Senator Bob Corker, a former mayor of Chattanooga.

Corker said that a fifth Marine is "fighting for his life" after the shootings.

Abdulazeez was killed after a gun battle with Chattanooga police near the scene of the murders, the U.S. Naval Reserve Center on Amnicola Highway near the Tennessee River.

The slaying of Abdulazeez has apparently ended the crisis here.

"As far as we know as this juncture there are no safety concerns for the general public," Killian said.

Local authorities would not reveal the names of the slain Marines. That will be handled by the Department of Defense.

Though the wounded Chattanooga police officer's name was revealed via social media earlier in the day, the officer has asked for his name not to be announced because of privacy concerns.

Chattanooga Police Chief Fred Fletcher said the officer was doing "as well as can be expected after being shot by a brazen criminal like this."

Reinhold confirmed that a residence in the Colonial Shores neighborhood of Chattanooga, in the suburb of Hixson, was being searched for evidence. The home reportedly belongs to the father of Muhammad Youssef Abdulazeez. The elder Adbulazeez was once under investigation by the FBI for possible links to terrorist groups, but no evidence was found, according to The New York Times.

Though TV footage showed a woman in traditional Middle Eastern attire being led from the house in handcuffs, Reinhold said no others were charged in the crime. Subduing people in such situations is done to provide more security for investigators.

What Corker termed "a long and sad day" began with Abdulazeez opening fire at a recruiting center on Lee Highway in the eastern part of the city. Firing from a rented convertible Mustang, he unleashed a steady spray of bullets; on Thursday evening, the parking lot had nearly two dozen blue cups marking the spots where empty shell casings were found. The recruiting center, which sits between a mobile phone store and an Italian restaurant, was the apparent target. However, there were no fatalities at that scene, with reports of only the wounding of a recruiting officer.

The four Marines were killed, and three others wounded, at the training center some 30 minutes later.

CNN reported that sources said Abdulazeez was using an AK-47 automatic weapon.

Abdulazeez was reportedly born in Kuwait but was a naturalized U.S citizen. He attended Red Bank High School, where he was a wrestler, and graduated from the University of Tennessee at Chattanooga with a degree in electrical engineering. He had worked an internship with TVA but had been unemployed.