STONYCREEK, Pa. — The grassy field where United Airlines Flight 93 crashed 17 years ago has become "a monument to American defiance," President Trump declared Tuesday, casting a memorial here as a message to the world: The United States won't give in to tyranny.

"A piece of America's heart is buried on these grounds, but in its place has grown a new resolve to live our lives with the same grace and courage as the heroes of Flight 93," Trump told several hundred people during an annual observance near Shanksville, Somerset County.

In a 15-minute speech, Trump honored the passengers and crew who resisted a hijacking during the Sept. 11, 2001, terrorist attacks that killed nearly 3,000 people. His appearance was the first presidential visit to the Flight 93 National Memorial since 2011, when President Barack Obama paid his respects.

Trump invoked personal stories about the flight, which never reached the terrorists' intended target in Washington, after passengers and crew members staged a revolt against their attackers. Forty people on board "took their fate — and America's fate — back into their own hands," the president said.

The group mounted their counterassault after hearing that other planes had struck New York's World Trade Center. Another plane struck the Pentagon.

"They boarded the plane as strangers," Trump said of the Flight 93 travelers, "and they entered eternity linked forever as true heroes."

He called them a "band of brave patriots" who changed history and commanded their own destiny. Free people "are never at the mercy of evil," the president said, "because our destiny is always in our hands."

His speech capped the roughly hour-long ceremony on muddy grounds at the memorial. Dignitaries gathered there Sunday to dedicate the Tower of Voices, a 93-foot concrete tower that will eventually hold 40 wind-activated chimes. Each will carry a unique ring tone to represent the distinct voices of those aboard Flight 93.

"They did not sit back and let events unfold for them," said Gordon Felt, president of the Families of Flight 93 organization. He urged Americans to remember what happened — and to rise up themselves.

Former Pennsylvania Gov. Mark Schweiker offered a similar reflection, reminding the crowd that college students are too young to have firsthand memories of the attacks. More than 6,200 days have passed since then, he said.

"The threats remain very real, so we must remain vigilant, and we must prepare a new generation of leadership," Schweiker said. It was in Somerset County "that freedom took its dramatic stand," he said.

Other speakers included Gov. Wolf, U.S. Interior Secretary Ryan Zinke, and the Rev. Paul Britton, whose sister Marion was among those on Flight 93.

"They challenge us to live strong lives, courageous lives, for others," Britton said.

Secret Service, National Park Service, and Pennsylvania State Police kept a heavy security presence at the memorial, where some officers appeared on horseback. Trump, accompanied by first lady Melania Trump, traveled by helicopter to the site after flying on Air Force One to John Murtha Johnstown-Cambria County Airport.

After the event, the first couple toured a viewing area near the crash site, where a Park Service guide described the area for them. They stood together and looked silently at the spot.

Earlier, their arrival energized the crowd, with many visitors wearing Trump campaign hats or shirts. Some whistled or cheered at the president's introduction on stage.

Beverly Naugle, 68, of Ligonier, said the president's strongest comment came in a vow to protect the country against more attacks.

"I thought he had an excellent speech. It was meaningful and genuine," said Naugle, who identified herself as a Trump voter. "I think that's who he is and why he got elected."

Her sister-in-law, Pattie Markovic, 64, of Greensburg, said the assembly made the anniversary more personal.

"You see things on TV, but being here — you can see the families and listen. It brings you closer to the families," she said.

For Rose Molanick, 58, of West Mifflin, the gathering brought peace. It was her first time at the memorial, where she said speakers had reflected poignantly on the deceased.

"They spoke very well — nothing political," she said. "It was all about the victims."