A security guard at Washington's Union Station on Friday fatally shot a man who had allegedly stabbed a woman in what police believe was an incident of domestic violence.

"We believe this is a domestic-related incident," Metropolitan Police Department's Jeff Brown told reporters. "Obviously with this being 9/11 fears were heightened."

The suspect died at the hospital, police said. The woman was being treated for her injuries.

Security had been beefed up around the U.S. capital on the anniversary of the Sept. 11, 2001, attacks on the World Trade Center twin towers in New York and the Pentagon.

Union Station, a popular tourist destination and the second busiest rail station in the United States, is only half a mile (1 km) from the U.S. Capitol building.

But Brown said the attack was an isolated incident.

Amtrak, which serves about 5 million train passengers annually at the station, said it had resumed operations "after a brief suspension due to police activity."

Police said the woman was in a passageway connected to the train station when the man stabbed her.

A security contractor for the Securities and Exchange Commission, which has its national headquarters next door to the station, then chased the man a short distance before the man turned on him with the knife. The guard fired once, hitting the man in the leg.

After the shot was fired some people temporarily evacuated while others "sheltered in place," Brown told reporters.

As well as providing a terminal for Amtrak and Maryland's commuter rail, the station has a busy subway stop and a depot for bus lines. It also houses many stores and restaurants and connects to some neighboring businesses and federal agencies through passageways.