The phrase "radical Islamic terrorism" is controversial, because it invokes the entire Muslim faith, and not just al-Qaeda, the terrorist group that committed the 9/11 attacks. Trump did not use the phrase last year in his commemoration of 9/11.

Trump also noted that since 9/11, "five and a half million young Americans have enlisted in the United States armed forces. Nearly 7,000 service members have died facing down the menace of radical Islamic terrorism."

"We will remember that free people are never at the mercy of evil because our destiny is always in our hands. America's future is not written by our enemies. America's future is written by our heroes," the president said.

"A piece of America's heart is buried on these grounds," Trump told guests at a memorial service in Shanksville, Pennsylvania, at the site of the crash of United Airlines Flight 93 on Sept. 11, 2001. "This field is now a monument to American defiance. This memorial is a message to the world: America will never, ever, submit to tyranny," Trump said.

President Donald Trump on Tuesday told the families of the victims of Flight 93 that their loved ones "stopped the forces of terror."

But this year, both the Trump White House and the Trump presidential campaign seemingly made a point of including the phrase.

On Tuesday, Michael Glassner, the Trump campaign's chief operating officer, said in a statement, "This day reminds us of the need for America's vigilance against radical Islamic terrorism and its ongoing threat to our freedoms, as President Trump has done so honorably as our Commander-in-Chief."

The flight was hijacked by four al-Qaeda members, who may have been planning to fly it into the U.S. Capitol. As passengers and crew sought to regain control of the plane, it crashed into a field outside of Shanksville. All 44 people aboard were killed.

A White House statement Monday marking Patriot Day used the phrase "radical Islamic terrorists," and not al-Qaeda to refer to the hijackers.

By the time Trump took the stage in Shanksville, however, he already had appeared to undermine the solemnity of the day. Trump began Tuesday morning by raging against his Justice Department in a series of tweets. Later, on the way to Pennsylvania, the president gave photographers a thumbs up outside Air Force One, and upon landing, he pumped both fists at his cadre of greeters.

Trump has a complicated history with the 9/11 attacks. On the day of the actual attacks, he was asked by a local TV station about a building he owned at 40 Wall Street. Instead of reflecting upon the loss of life just hours earlier, Trump falsely claimed that with the World Trade Center having collapsed, his own building was now "the tallest" in the city.

In fact, a nearby building on Pine Street was 25 feet taller. Trump described how, according to his staff near the World Trade Center, "Wall Street is littered with two feet of stone and brick and mortar and steel."

Trump has also used the 9/11 attacks to stoke unfounded fear of Muslims. During Trump's 2016 presidential campaign, he also falsely claimed to have seen video of Muslims in New Jersey celebrating the collapse of the towers.

"There were people that were cheering on the other side of New Jersey, where you have large Arab populations," Trump said on ABC's "This Week" in late 2015. "They were cheering as the World Trade Center came down. I know it might be not politically correct for you to talk about it, but there were people cheering as that building came down — as those buildings came down," Trump said.