SAN FRANCISCO, May 6 (Xinhua) -- The Seattle-Tacoma International Airport (Sea-Tac Airport) in the U.S. West coastal state of Washington was forced to close its departure terminal because of a suspicious package Sunday, airport authorities said.

The upper drive at Sea-Tac Airport was evacuated Sunday afternoon after a report of the suspicious package, and the Seattle police department is investigating the incident, it added.

"The Port of Seattle Police Dept is responding to a suspicious package outside terminal. Out of an abundance of caution, the upper drive has been evacuated. Passengers may be picked up and dropped off in the airport parking garage for free," Sea-Tac Airport@SeaTacAirport tweeted.

"Passengers need to enter the airport from skybridges 1, 2 or 3 at the south end of the airport. Traffic is extremely slow. To avoid lengthy delays, we also encourage passengers to use Link light rail - drop off at Tukwila, Angle Lake or other nearby stations," it said.

Traffic nearby is backing up and it is at a complete stop approaching the departure terminal at the Sea-Tac airport, the primary airport serving the Seattle metropolitan area, which is about 21 kilometers south of downtown Seattle, the largest city in Washington state.

It's not known how and by whom the package was left at one of the most bustling airport in western U.S. area.