NEW YORK (AP) -- A commemoration of the 15th anniversary of 9/11 took place at ground zero Sunday.
The ceremony began with a moment of silence and tolling bells at 8:46 a.m., the time when a terrorist-piloted plane slammed into the World Trade Center's north tower.
After the silent period, victims' relatives began reading the names of the nearly 3,000 people killed when four hijacked aircraft hit the trade center, the Pentagon and a Pennsylvania field. It was the deadliest terror attack on American soil.
President Barack Obama will speak at an observance at the Pentagon.
Democratic presidential nominee Hillary Clinton and Republican rival Donald Trump are attending the anniversary ceremony at the World Trade Center.
Hundreds are also gathering in Pennsylvania where one of the planes hijacked by terrorists crashed in a field 15 years ago.
They gathered at Flight 93 National Memorial in Shanksville as ceremonies were also taking place Sunday in New York and at the Pentagon. Forty passengers and crew members died in the Pennsylvania crash.
For the first time, the Shanksville ceremony is being held outside the visitor center that opened this year rather than at the granite mall that runs along the crash site. The names of the victims will be read and bells ring in their memory.
The United Airlines flight was heading from Newark, New Jersey to San Francisco when it crashed after passengers and crew members fought the terrorists for control of the plane.