Roses line the north memorial pool at the 15th Anniversary of September 11 at the 9/11 Memorial.
Family members of those lost on September 11 are arriving at ground zero for the ceremony marking the 15th anniversary of the attacks.
Some are going into National September 11 Museum, which is open only to victims' families until Sunday afternoon, when the public will be allowed to enter.
About an hour before the official commemoration start, police officers in uniform and bagpipers rehearsed part of the ceremony involving a display of the US flag.
As it has every year, the remembrance will mainly focus on the reading of the names of those killed in the attacks. It will also include moments of silence and the tolling of bells.
Other ceremonies are being held at the Pentagon and at the Flight 93 National Memorial in Shanksville, Pennsylvania.