The United States marked the 13th anniversary of the Sept. 11 attacks against a troubling backdrop. The ceremonies came just hours after President Barack Obama told the country he is authorizing stepped-up airstrikes in Iraq and Syria against Islamic State extremists.

In New York City, family and friends of those who died read the names of the nearly 3,000 people killed there, at the Pentagon and near Shanksville, Pennsylvania. In Washington, Obama observed a moment of silence outside the White House with the first lady and Vice President Joe Biden before heading to the Pentagon's 9/11 memorial.

Here's a gallery of photos from Thursday's commemoration.

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