NEW YORK (AP) — The 9/11 anniversary commemoration at ground zero began Saturday with a tolling bell and a moment of silence, exactly 20 years after the start of the deadliest terror attack on U.S. soil.

President Joe Biden, former presidents Barack Obama and Bill Clinton joined a crowd of victims’ relatives and first responders at the Sept. 11 memorial plaza in New York. The memorial stands where the the World Trade Center’s twin towers were rammed and felled by hijacked planes.