US marks 21st anniversary of September 11 terror attacks

Smoke and flames erupting from the twin towers of New York City's World Trade Center after the attacks on September 11, 2001. [AP Photo]

Americans are remembering 9/11 with moments of silence, readings of victims' names, volunteer work and other tributes 21 years after the deadliest terror attack on U.S. soil.

Victims' relatives and dignitaries will convene Sunday at the places where hijacked jets crashed on Sept. 11, 2001 - the World Trade Center in New York, the Pentagon and a field in Pennsylvania.