(AP) -- President Barack Obama is joining the nation in remembering the nearly 3,000 people who died in the Sept. 11 attacks 15 years ago.

The White House says Obama observed the somber anniversary with a moment of silence in the Oval Office at 8:46 a.m. EDT. That's the time when the attacks began on that sunny day in 2001 -- when a hijacked airplane slammed into the north tower of New York City's World Trade Center.

Afterward, Obama arrived at the Pentagon, where he laid a large wreath at the beginning of a memorial service.

The American flag is flying at half-staff atop the White House and other federal buildings. Obama has invited governors, interested organizations and individuals to follow suit.

Nearly 3,000 people were killed in New York, Pennsylvania and at the Pentagon when al-Qaida terrorists crashed the airplanes in those locations.

U.S. forces killed al-Qaida leader Osama bin Laden nearly a decade later during a May 2011 raid that Obama authorized on his Pakistani hideout.