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Joseph Kaczmarek, Associated Press
Audience members gather before the start of a commissioning ceremony for the USS Somerset (LPD 25) Saturday, March 1, 2014, in Philadelphia. The USS Somerset is the ninth San Antonio-class amphibious transport dock and the third of three ships named in honor of those victims and first responders of the attacks on the World Trade Center and the Pentagon. The ship is named for the county where Flight 93 crashed after being hijacked on Sept. 11, 2001.

PHILADELPHIA — A new U.S. Navy ship named to honor 40 passengers and crew killed when their hijacked United Airlines flight crashed as they fought with terrorists during the Sept. 11 attacks has been put into service in Philadelphia.

About 5,000 people attended the ceremony commissioning the USS Somerset on Saturday.

The ship is named for the western Pennsylvania county where Flight 93 crashed. The amphibious transport dock warship is the third ship to be named in honor of 9/11 victims. The USS New York and USS Arlington honor those killed in the World Trade Center towers and the Pentagon during the attacks.

Republican Sen. Pat Toomey says the ship doesn't commemorate "an attack on America" but rather "the day America began to fight back."