In pictures: 9/11 ceremony

The ceremony was held under a brilliant blue sky reminiscent of the day of the attacks four years ago.

A member of the New York City Fire Department rang a bell to mark moments of silence.

The pain of the loss is still keenly felt by relatives and friends four years on.

One woman wept over fire chief Louis Joseph Modafferi, who died when the towers collapsed.

Many of those who lost relatives or loved ones were overcome with emotion.

Mourners brought flowers to place in two special pools in memory of those lost.

People made the journey down a ramp to the "footprint" of the former World Trade Center.

The commemoration came as the US was dealing with the pain of Hurricane Katrina's destruction in southern states.

The names of nearly 3,000 people killed in the attacks were read out in front of grieving relatives.

US Secretary of State Condoleezza Rice met with police - colleagues of some of those killed in the 2001 attacks.

so sad.. Sad