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Airstrikes in Iraq, unrest in Syria and the beheadings of two US journalists cast a shadow over the 13th anniversary of 9/11.
America is marking the 13th anniversary of the September 11 attacks against the backdrop of a new Islamist threat rising in the Middle East.
The commemorations began with a moment of silence, the toll of a bell, and then the names of the nearly 3,000 victims who died in the attacks were read out.
On the eve of the anniversary, President Barack Obama had laid out his strategy on how to confront Islamic State militants, telling Americans of a sustained effort needed against a growing threat.
The memorial service in New York is the first since the opening of the 9/11 museum last May.
One World Trade Center has risen 1,776 feet above ground zero, and rebuilding efforts at the site, where 2,753 people died, are nearing completion.
"The memorial and museum is extremely important to those impacted on 9/11," said Mary Fetchet, whose son died in the attacks.
"And surrounding that memorial, lower Manhattan has been revitalized."
The moments of silence and recitations of the victims' names has become an annual ritual.
Similar ceremonies will also be held in Washington, where a hijacked plane crashed into the Pentagon, and in the field in Shanksville, Pennsylvania, where another hijacked plane crashed.
Mr Obama is expected to speak during a private ceremony at the Pentagon on Thursday morning for relatives of the those who were killed there.