A Pentagon police officer was stabbed in an attack at the Pentagon Transit Center Tuesday morning and later died, two law enforcement sources told ABC News.

The officer’s assailant, identified as Austin William Lanz of Georgia, also died as a result of the incident, the law enforcement sources said.

He stabbed the officer in the head from behind, then disarmed the officer and proceeded to shoot the officer with this own gun, according to law enforcement sources. The assailant was likely killed by a gunshot wound, the sources said, but it was unclear whether he shot himself or was killed by police.

At least one other bystander was injured, according to those law enforcement sources.

The FBI is leading the investigation into the attack.

Lanz enlisted in the Marine Corps on Oct. 9, 2012 but was administratively separated on Nov. 2, 2012 and never earned the title Marine, according to the Marine Corps.

He was arrested in Cobb County, Georgia, in April on several charges — including aggravated battery against police, rioting in a penal institution and making terrorist threats or acts, according to county online court records. The judge in that case ordered Lanz have a mental health and substance abuse evaluation and he was released on a $30,000 bond.

The FBI Atlanta office is also conducting an investigation into Lanz, according to the law enforcement sources.

The Pentagon Force Protection Agency confirmed the officer’s death in a tweet Tuesday evening, but withheld additional information pending full next of kin notification.

Defense Secretary Lloyd Austin, in a statement released a short time after the officer’s death was announced, offered his condolences and said that he’d ordered flags flown on the Pentagon Reservation be flown at half staff.

“This fallen officer died in the line of duty, helping protect the tens of thousands of people who work in — and who visit — the Pentagon on a daily basis. He and his fellow officers are members of the Pentagon family, and known to us all as professional, skilled and brave. This tragic death today is a stark reminder of the dangers they face and the sacrifices they make. We are forever grateful for that service and the courage with which it is rendered,” the statement said.

Chairman of the Joint Chiefs of Staff Gen. Mark A. Milley also released a statement Tuesday that said, “On behalf of the entire Joint Force, I extend our deepest condolences to the family, friends and Pentagon Force Protection Agency colleagues of the officer who was killed today in the line of duty protecting the thousands who work at and visit the Pentagon each day. This officer’s bravery will always be honored.”

Chief Woodrow Kusse, who leads the Pentagon Force Protection Agency, joined Pentagon spokesman John Kirby at an afternoon press briefing to address the incident, but he would not provide details about casualties, including whether an officer was even wounded.

“This morning at about 10:37 a.m., a Pentagon police officer was attacked on the Metro Bus platform. Gunfire was exchanged. And there were — there were several casualties. The incident is over, the scene is secure and — most importantly — there’s no continuing threat to our community,” he said.

“There were a number of people that fled and there were some erroneous reports,” he added.

While sources told ABC News there was no known motive, they added that there was no obvious connection to terrorism. Those same sources stressed it’s still early in the investigation.

The medical examiner in Washington did not immediately respond to a request for a comment.

Pressed on reports on whether an officer died, Kusse said Tuesday afternoon that he couldn’t release those details as the investigation is ongoing.

“I don’t want to compromise the integrity of that process right now,” he said.

“I’m not confirming or denying those particular reports right now the investigation is ongoing. And I do promise to get back as soon as possible, with further details but I can’t release those right now,” he said.

Pressed for information about the assailant, he added, “We are not actively looking for another suspect.”

In his statement, Austin also reminded that the investigation into the circumstances of the incident was underway.

“We must let that investigation proceed unimpeded and without speculation,” Austin said in the statement. “In the meantime, we will keep the family of our fallen officer — and his fellow officers — foremost in our mind and provide them whatever support they require. I know nothing we can say will properly assuage their grief, but I hope they know we mourn with them.”

Earlier Tuesday on the National Mall, Capitol Police officers on motorcycles led a ceremonial procession, passing by saluting law enforcement officers from Washington’s Metropolitan Police Department, the Pentagon Force Protection Agency and other police departments, to honor the Pentagon police officer.

The Pentagon was placed on lockdown Tuesday morning after the incident at the Pentagon Transit Center involving a stabbing and a shooting, according to a U.S. official.

The lockdown was later lifted and the Pentagon reopened, the Pentagon Force Protection Agency said shortly after noon.

The Pentagon had no details regarding the assailant’s motivation Tuesday afternoon, but Kusse said they will review the results of the investigation before making a determination on whether security measures should change.

“Every time an incident occurs, whether it’s here or anywhere else across the nation or in the world, we do after actions on those we examine them, we look for things that we can do to improve. But right now, again, it’s still pending, we will certainly, as this investigation concludes, take another look at any measures,” he said.

Austin and Milley were not in the Pentagon at the time of the incident. They were both at the White House for their weekly meeting with President Joe Biden and they were all aware of the ongoing situation.

Kirby said that Austin was back in the Pentagon Tuesday afternoon and had a chance to visit the Pentagon police operations center to check in and express his gratitude for their work.

ABC News’ Libby Cathey and Matt Seyler contributed to this report.