CHATTANOOGA (KRON/AP) — President Barack Obama is promising a thorough and prompt investigation into the attack at two military sites that killed at least four Marines in Chattanooga on Thursday morning.

Obama says it appears the attack was committed by a lone gunman, but that a full investigation is ongoing.

The president is calling the shooting a “heartbreaking circumstance.” He says he’s been briefed about the incident and has been in touch with the Defense Department to make sure military facilities are vigilant.

“As we have more information, we’ll let the public know,” Obama said. “I want everybody to understand that we will be thorough and prompt in our figuring out exactly what happened.”

Obama spoke in the Oval Office just minutes after returning to Washington from a trip to Oklahoma. He was joined his counterterrorism and homeland security adviser and the FBI director.

The shootings began at the recruiting center on Old Lee Highway, where a shot rang out around 10:30 or 10:45 a.m., followed a few seconds later by more fire, said Sgt. 1st Class Robert Dodge, the leader for Army recruiting at the center.

The alleged gunman in the shootings was identified as Muhammad Youssef Abdulazeez, CNN is reporting, citing two sources briefed on the case. He was born in 1990, a law enforcement official said.