(AP) -- A man stabbed a woman and then was shot by a security guard at Washington's massive Union Station on Friday, the anniversary of the September 11 attacks, District of Columbia police said.

The guard saw the man stabbing the woman and chased him, police spokesman Gwendolyn Crump said in a statement. The man turned and pointed the knife at the security guard, who opened fire, and the man was shot, she said.

The stabbing was believed to be the product of a domestic dispute, Crump said.

One ambulance was seen leaving the scene with its lights and sirens on, followed by a police car. The D.C. fire department said it had taken two patients from the scene -- one in critical condition and one whose condition was not known.

Union Station is a massive transit hub and home to Amtrak's headquarters. The station is bigger than the nearby U.S. Capitol and serves as the southern terminus for passenger rail traffic in the northeast. It includes dozens of retails shops and restaurants, which typically draw thousands of midday diners.

Some 90,000 people pass through Union Station each day, according to Amtrak.

Flags on the towering poles outside the station's main entrance were flying at half-staff Friday in remembrance of the terrorist attacks on Sept. 11, 2001.