Four U.S. Marines were killed and two other servicemen were wounded Thursday in shootings at two military facilities in the southern city of Chattanooga, Tennessee.
The gunman who opened fire at the two facilities was also killed.
Law enforcement identified the shooter as Muhammad Youssef Abdulazeez, 24, of Hixson, Tennessee, just across the Tennessee River from Chattanooga.
Authorities say he opened fire in a drive-by shooting at a military recruiting center at a shopping mall in Chattanooga where five branches of the military all have adjoining offices.
The U.S. Defense Department said 25 to 30 shots were fired and one Marine Corps recruiter suffered a leg wound. He was treated at a local hospital and released.
Witnesses said the shots began about 10:30 a.m.Sgt. 1st Class Robert Dodge, 36, the leader for Army recruiting at the center, said he and his colleagues then got on the ground and barricaded themselves in a safe place.
Dodge said he did not see the shooter or a vehicle.
The Army recruiting office was not damaged, but doors and glass were damaged at the neighboring Air Force, Navy and Marine offices, he said.
The gunman next drove about 10 kilometers away to the Navy Operational Support Center (NOSC), where witnesses say he unleashed a barrage of gunfire.
The four Marines were killed at that center, and a sailor was wounded. Reported to be seriously injured, he is still undergoing treatment.
The shooter was killed by local law enforcement officials, who chased him from the first location.
The names of the dead Marines were withheld pending notification of relatives.
Law enforcement officials said that Abdulazeez was not immediately found on any terrorism databases.
Born in Kuwait, Abdulazeez was a naturalized U.S. citizen. He was a high school wrestler with a degree in engineering from the University of Tennessee in Chattanooga.
The SITE Intelligence group reports that Abdulazeez wrote a blog and on Monday he said that "life is short and bitter."
He added, according to SITE, that Muslims should not miss "the opportunity to submit to Allah."
Last April, local police stopped Abdulazeez for erratic driving and for speeding, according to court documents. He was charged with driving under the influence.
But a woman who attended Red Bank High School with Abdulazeez told the Chattanooga Times Free Press that he was a quiet kid and well-liked.
"He was friendly, funny, kind," Kagan Wagner told the newspaper. "I never would have thought it would be him.
"They were your average Chattanooga family," she added.
The New York Times reported Abdulazeez's father was under investigation several years ago for possible ties to a foreign terrorist organization.
Pentagon spokesman Col. Steve Warren told reporters the gunman would have had no trouble gaining access to the military facilities.
"We have our recruiting centers set up in places easily accessible to the public – for example, a strip mall. So security there is not like we would see at a Fort Bragg or at a Norfolk Naval Air Station or at Quantico [Virginia]. So this is something that we have to face," Warren said.
He added, "This is because we have to be in contact with the American public."
The Navy Operational Support Center, which sits in a light-industrial area, is used by Navy and Marine Corps personnel, and is often referred to as a "reserve center." The two entrances to the fenced facility have unmanned gates.
"The NOSC Chattanooga mission is to provide training and readiness support for our reserve component personnel to enable them to support the needs of the Navy and Marine Corps team," a Defense Department statement read.
U.S. authorities are increasing security at federal facilities in the wake of the shootings.
Homeland Secretary Secretary Jeh Johnson said his department is closely monitoring the shooting and the FBI investigation, but he cautioned, "there are many unconfirmed and possibly false reports about events."
Defense officials said that because the gunman did not set foot on federal military property, the investigation of the shootings falls under the authority of local law enforcement and the Federal Bureau of Investigation.
The FBI, which is taking the lead on the investigation with local law enforcement, said it is investigating the possible motive for the attack. The bureau has promised a thorough review.
But Bill Killian, U.S. attorney for the Eastern District of Tennessee, was more blunt. "We are treating this as an act of domestic terrorism," he said.
In a background briefing for reporters, a Defense official said chatter from terror cells had increased after the Islamic State group called for attacks during the Muslim holy month of Ramadan, but that none of the chatter had mentioned military recruiting centers. Ramadan ends Friday.
"Very publicly ISIL [Islamic State] had called on their fellow jihadis to commit acts of terror," an official said. "In association with that public statement by ISIL, we did see an increase in chatter. None of it was very specific. The FBI has announced that they made some arrests. We monitor to see what force posture to take."
Message of sympathy
From the Oval Office, President Barack Obama promised a "thorough and prompt" investigation, but said his main message right now is one of sympathy for the slain servicemen and their families.
"It is a heartbreaking circumstance for these individuals who have served our country with great valor to be killed in this fashion," he said.
"Today was a nightmare for the city of Chattanooga," Mayor Andy Berke said. "As a city, we will respond to this with every available resource that we have."
"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.
Located in the southeastern part of Tennessee, Chattanooga is the fourth largest city in the state.
Tucked between the Blue Ridge Mountains and the Tennessee River, it is known as the Scenic City and is considered a travel destination.
Some information for this report came from Reuters and AP.