Americans paused Wednesday to mark the 18th anniversary of the September 11, 2001 terrorist attacks that killed nearly 3,000 people in New York, Virginia and Pennsylvania.

President Donald Trump and first lady Melania Trump walked outside the White House to observe a moment of silence at 8:46 a.m., the time when hijackers flew the first of two commercial jetliners into the World Trade Center in New York.

Later, Trump is addressing a ceremony at the Pentagon for families of those killed when al-Qaida terrorists flew a hijacked plane into the building that is the headquarters of the U.S. military. Early Wednesday, ahead of the ceremony, an American flag was unfurled down the side of the Pentagon.

In New York, in an annual ceremony at Ground Zero of the attack, people began reading the names of all the victims killed on the day that now is etched in the memories of Americans across the country. Hundreds of survivors and family members of those killed gathered at the 9/11 Memorial, 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 attending ceremonies in Shanksville, Pennsylvania, near where United Airlines Flight 93 crashed after passengers took control from the terrorists who had hijacked the plane with the intention of crashing it into the White House.

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 September 11 permanently changed America's perception of national security and prompted then-President George W. Bush to declare war on terrorism and invade Afghanistan, where al-Qaida had training grounds.

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, 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 Abbottabad, 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.