US Navy Yard, Washington, DC

At least 13 people were killed and several others wounded in a shooting rampage at the Navy Yard in Washington, D.C., Monday, CNN reports. The death toll included one suspect, whom the FBI later identified as 34-year-old Aaron Alexis of Fort Worth, Texas. "We have no indication of motive at this time," Washington D.C. police chief Cathy Lanier told reporters.

Alexis was a former Navy electrician's mate who was discharged from the service in 2011 after four years because of multiple disciplinary infractions, according to The Los Angeles Times. A Navy official told the newspaper that Alexis had a "pattern of misconduct."

Although police said Monday afternoon that there may be two other accomplices on the loose, authorities later established that one of them was not a suspect in the shooting. "We don't know that there is a second gunman on the loose," Washington D.C. Mayor Vincent Gray told reporters. Nonetheless, police were still searching for another possible gunman wearing military-style clothing as of late Monday — described as a black man in his 50s dressed in olive fatigues that witnesses had reported seeing.

Fall TV Preview: Must-watch new shows

"We are confronting yet another mass shooting," President Barack Obama said just hours after the attack, according to CNN. "And today it happened on a military installation in our nation's capital."

Alexis is thought to have used another man's identification to pass through the gates on Monday morning.  According to the Times, Alexis then headed for Building 197, the Navy Sea Systems Command headquarters, armed with three weapons. He went to the fourth floor and then began shooting down a crowded atrium at approximately 8:15 a.m., targeting military personnel and civilians. The surrounding area was put on lockdown for several hours after the initial shots were reported, and air traffic was briefly suspended at nearby Reagan National Airport in Virginia.

About 3,000 people work at the headquarters, CNN reports.