Americans on Wednesday are marking the 18th anniversary of the Sept. 11, 2001, terrorist attacks that killed almost 3,000 people in New York, Virginia and Pennsylvania.

President Donald Trump is observing the day with a morning moment of silence at the White House before attending a ceremony at the Pentagon for families of those killed when al-Qaida terrorists flew a hijacked plane into the building.

In New York, hundreds of survivors and family members of those killed are gathering at Ground Zero, where the Twin Towers of the World Trade Center stood before two al-Qaida-hijacked commercial flights brought them down.

Vice President Mike Pence is scheduled to attend ceremonies in Shanksville, Pennsylvania, near where United Airlines Flight 93 crashed after passengers took control from the terrorists who had hijacked the plane.

Nineteen men affiliated with al-Qaida carried out the four hijackings.

The deadliest attack on American soil since Pearl Harbor in 1941, the events of Sept. 11 permanently changed America's perception of security and prompted then-President George W. Bush to declare war on terrorism and invade Afghanistan.

U.S.-led coalition forces knocked the Taliban from power in Afghanistan, but the conflict is still ongoing, ranking as the United States' longest war. U.S. and Taliban representatives have held recent peace talks, but just as details of a potential agreement were being made public, President Donald Trump canceled planned meetings with Taliban and Afghan officials because of continued Taliban attacks in Kabul.

The United States searched for al-Qaida leader Osama bin Laden for years after the attacks, and in May 2011 a team of U.S. Navy Seals raided a compound in Abbottabod, Pakistan, where they shot him dead.

Memorials for those killed in the attacks now stand at all three sites in New York, Virginia and Pennsylvania. The Pentagon crash site was quickly rebuilt after the attacks. A new tower at the World Trade Center site took longer to construct, but now rises above the Manhattan skyline as the tallest building in the United States.