The U.S. Capitol and its office buildings were briefly locked down Tuesday after suspects nearby shot at District of Columbia police with a small machine gun.

Police arrested three men after stopping their car about two blocks from the Capitol, Police Chief Cathy Lanier said.

Police were alerted to a man with a gun near Fort McNair in southwest Washington, about a mile and a half south of the Capitol, shortly after 4 p.m., police spokesman Dustin Sternbeck said.

Lanier said officers followed the car into a tunnel that goes under the National Mall near the Capitol.

As the car was entering the tunnel, Lanier said, someone fired at the officers with what she called “a small machine gun.”

No officers fired their weapons, Lanier said. “They were in the middle of a crowded tunnel and that was not an option at that point,” said Lanier, who said the gun was dropped in the tunnel and recovered by police.

Police also said found a large amount of ammunition.

Capitol Police later reported: “Two of the suspects were arrested and taken into custody on the scene. One of the three suspects fled the scene, causing the lockdown until the suspect could be apprehended. Once the suspect was located and taken into custody, the lockdown was lifted.”

The Capitol lockdown began at 4:20 p.m. and the all-clear was given over a loudspeaker at 4:45 p.m. Police had sent an email to congressional staff advising them to close and lock doors and stay away from external doors and windows.

Both the House and Senate were in session, with House members voting. In the Senate, lawmakers were delivering speeches from the floor.

Last Friday, the U.S. Capitol was briefly locked down after reports that a woman with a gun was attempting to enter the building. It turned out to be a false alarm.

(AP)