An armed security guard at Walter Reed Army Medical Center began firing at another guard during an argument Wednesday outside a busy entrance to the facility, District of Columbia police said. No one was injured.

The argument began about 8:30 a.m., police said, prompting one of the guards to pull out his gun and fire as many as 10 shots, Lt. Jimmie Riley said.

The other guard fled the scene without getting hit and ran to a nearby house to place a call to police, Riley said. The guard who fired the shots was taken into custody.

"We're very fortunate," Riley said. "It could've been tragic."

The two guards are employed by a private security firm, which is under contract to Walter Reed.

The hospital in Washington is the Army's premier center for treating injured soldiers returning from Iraq and Afghanistan.