The gunman who opened fire on two military centers in Chattanooga Thursday, killing four Marines and critically wounding a Navy sailor, was not in any federal terrorism database and was not under investigation before he carried out the rampage, several officials told NBC News.

Mohammad Youssuf Abdulazeez, 24, was killed after spraying dozens of bullets at a military recruiting center, then driving to a Navy-Marine training center seven miles away, where he got out of his car and shot the Marines. A police officer was also wounded and was being treated in the hospital, officials said.

Three of the slain Marines were identified Friday as Thomas Sullivan, 40, originally from Springfield, Massachusetts; Skip Wells, 21, of Marietta, Georgia; and David Wyatt, originally from Burke, North Carolina.

A motive in the attack is unclear, but the FBI is investigating a disturbing blog that may yield clues. The SITE Intelligence Group said Abdulazeez posted three days before the shooting that "life is short and bitter" and the Muslims should not let "the opportunity to submit to Allah ... pass you by." The blog has not been confirmed by NBC News.

Investigators believe Abdulazeez, who had several weapons, acted alone. A federal law enforcement source told NBC News that Abdulazeez rammed the fence at the second location, the naval reserve center, and fired at officers with an AK-47-style weapon. It is not yet clear if he was killed by police or if he killed himself.

Chattanooga Police Chief Fred Fletcher told reporters late Thursday, "I can tell you that the officers of the Chattanooga police department saved many lives today."

Officials suspect the attack may have been ISIS-inspired because of the target of the attack and the timing — the Muslim holy month of Ramadan ended Thursday — but investigators have not confirmed that the attack was inspired by the Islamist terror organization.

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Law enforcement officials say there are "other indications" it may have been ISIS-inspired, but they declined to be specific.

"We will explore that option, but at this point, we don’t have anything that directly ties him to an international terrorist organization," FBI Special Agent in Charge Edward W. Reinhold told reporters late Thursday. "We have no idea at this point what his motivation was behind the shooting."

He added: "We are checking every possible place where he could reside ... where he shopped, where he went to school, who his friends were, if he worked out at a gym, every possible lead."

Abdulazeez was born in 1990 in Kuwait, and he was a naturalized U.S. citizen. He lived with his parents, and his father is a Chattanooga city employee. The Kuwaiti Interior Ministry said Friday that Abdulazeez was of Jordanian origin and only born in Kuwait.

Abdulazeez was not under FBI surveillance and was not on any watch list, officials said. However, his father, who is from the Palestinian territories, was briefly investigated several years ago for possible connections to a terror organization. But that investigation was closed, and the father was removed from government watch lists, the officials said.

Abdulazeez was a 2012 graduate of the University of Tennessee in Chattanooga, where he earned a degree in electrical engineering, the university said. He was a student intern at the Tennessee Valley Authority about five years ago, the agency said.

Related: Marine in Chattanooga Rampage Describes Attack

Abdulazeez briefly signed up with a mixed-martial arts gym after high school, where the gym owner recalled that he "seemed like a normal kid back then."

"I haven't seen him in years, but I'm very shocked that it's him," Jesse Grun, who used to run the Chattanooga Fight Factory, told NBC news Thursday. He spent about six months at the gym, before his parents pulled him out.

Grun described him as a typical teenager. He wasn't especially loud, but wasn't shy, either. "I believe he was still an opened-minded young happy teenager, so something must have changed as he grew into an adult," Grun said.

In his Red Bank High School yearbook, Abdulazeez’s senior entry contained this quote: "My name causes national security alerts. What does yours do?"

He was arrested by Chattanooga police April 20 on a DUI charge. Officers said they smelled marijuana on him, and he appeared intoxicated, NBC station WRBC reported, and he was scheduled to appear in court at the end of July.

Meanwhile, the Council on American-Islamic Relations condemned the deadly rampage.

"Such inexcusable acts of violence must be repudiated by Americans of all faiths and backgrounds," CAIR National Executive Director Nihad Awad said. "The American Muslim community stands shoulder to shoulder with our fellow citizens in offering condolences to the loved ones of those killed and injured and in rejecting anyone who would harm our nation’s safety and security.