Four Marines were killed Thursday in shootings at a pair of military facilities in Tennessee by a gunman who is being investigated for possible ties to Islamist terrorist groups, U.S. law enforcement officials said.

The shooter, identified by the FBI as Mohammad Youssef Abdulazeez, 24, of Hixson, Tennessee, was killed, apparently by police, at the end of a rampage that also left a Chattanooga police officer and a member of the U.S. military wounded.

The assault marks the latest eruption of gun violence in the United States. It comes amid a flurry of recent arrests, disrupted plots and warnings from U.S. counterterrorism officials that the Islamic State terrorist group has called on its followers to mount attacks against U.S. targets, including military installations.

During brief remarks late Thursday at the White House, President Barack Obama described the Marines' deaths as a "heartbreaking circumstance for these individuals who have served our country with great valor." He avoided describing the attack as a terrorist plot, saying, "We don't know yet all the details" and that "a full investigation is taking place."

Even so, Obama described stepped-up security measures as White House officials urged the Pentagon to "make sure that all our defense facilities are properly attentive and vigilant as we sort through what happened."

U.S. officials said an initial check of federal terrorism databases produced no information that Abdulazeez had been under investigation.

The 6th Marine Corps Recruiting District, which includes Tennessee, closed all facilities within 40 miles of the shooting as a security measure.

Navy Secretary Ray Mabus said that "while we expect our sailors and Marines to go into harm's way, and they do so without hesitation, an attack at home, in our community, is insidious and unfathomable."

Authorities said they had reached no conclusions about the shooter's motivations. He had been arrested in April for driving under the influence. He appears to have made frequent postings about his religious views online. Public records indicate that Abdulazeez had no criminal history aside from vehicle-related offenses.

In a news conference in Chattanooga, Edward Reinhold, the special agent in charge for the FBI in Knoxville, declined to discuss details of the investigation. A senior U.S. law enforcement official said it appeared that police killed Abdulazeez as he fled.

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The attack began at a recruiting center, the Armed Forces Career Center, where five branches of the military have adjoining offices in a strip mall. Such recruiting sites are easily accessible to the public, defense officials said.

Some time between 10:30 and 10:45 a.m., the gunman drove up in a gray Ford Mustang, according to a federal law enforcement official. He was armed with multiple weapons and opened fire with a long gun, unleashing a barrage of more than two dozen rounds, the official said.

No one was reported hit at the first site, officials said, where police from several agencies responded to reports of the shots. The gunman fled by car and police gave chase to the Navy and Marine Corps Reserve Center on Amnicola Highway, less than a half-hour away.

There, the Ford convertible, with the top down, arrived and the gunman jumped out and "almost instantly" started firing, according to a federal source.

The four Marines were killed at the second center, federal officials said. A Marine recruiter was treated for a leg wound. Another injured man was described by hospital officials as being in critical condition. A police officer was wounded in the ankle.

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FBI Director James Comey recently disclosed that authorities had arrested more than 10 people as part of a burst of activity by law enforcement to prevent attacks timed to the July Fourth holiday as well as the Muslim observance of the month of Ramadan, which ends Friday.

With Los Angeles Times