On the 13th anniversary of al-Qaeda's attack on the World Trade Center towers, the Pentagon and the crash of United Flight 93, the nation remembers the attack that changed this nation's and the world's history.

Ceremonies were held in New York City, Washington, D.C. and Shanksville, Pennsylvania where local, state and federal leaders, including President Obama, remembered the attacks that killed thousands.

In lower Manhattan, relatives and friends of victims who died in the World Trade Center attacks gathered at the 9/11 Memorial to remember their loved ones who died during the attacks in 2011. The Memorial was closed to the public until 6 p.m. for the remembrance ceremony, according to the Associated Press.

Former New York mayors Rudy Giuliani and Michael Bloomberg as well as current mayor Bill de Blasio attended alongside New York Gov. Andrew Cuomo and New Jersey Gov. Chris Christie.

During the ceremony, relatives of survivors read out the names of the almost 3,000 office workers, EMS personnel, firefighters and police officers that died in the attacks and subsequent collapse of the towers.

In Washington, D.C., President Obama and the First Lady, Michelle Obama, along with White House personnel held a moment of silence on the White House's South Lawn alongside Vice President Joe Biden. The event was held at 8:46 a.m., according to NBC Washington.

The President then attended another commemorative event at the Pentagon where the third hijacked plane hit. The event was invitation-only and was attended by federal and military personnel as well as relatives of the victims and survivors. A moment of silence was held at 9:47 a.m. when the plane hit one of the Pentagon's sides.

According to USA Today, a commemorative event was also in Shanksville, where United Flight 93 crashed after being brought by its passengers. The Flight 93 National Memorial will be open to the public all day. A wreath-laying ceremony and a reading of names happened during the remembrance ceremony.