WASHINGTON ― President Donald Trump on Monday commemorated the 16th anniversary of the Sept. 11 attacks, leading a moment of silence at the White House to mark the moment the first plane struck the North Tower of the World Trade Center in New York.

The president and first lady Melania Trump stood on the White House’s South Lawn amid a crowd that included Trump’s elder daughter, Ivanka, and his son-in-law Jared Kushner.

Trump spoke at a memorial ceremony at the Pentagon later Monday morning, honoring the nearly 3,000 victims of the attacks and their families.

“Today, our entire nation grieves with you,” he said, reflecting on “the horror and anguish of that dark day” and praising the country’s perseverance and unity following the attacks.

His measured remarks on Monday notwithstanding, Trump has a history of making insensitive and false comments about the Sept. 11 tragedy.

The 2001 terrorist attacks were the subject of one of Trump’s most egregious lies during his presidential campaign. In November 2015, he claimed, without evidence, that “thousands and thousands” of Muslims in New Jersey had celebrated the news of the attacks.

“I watched when the World Trade Center came tumbling down. And I watched in Jersey City, New Jersey, where thousands and thousands of people were cheering as that building was coming down,” Trump said at a campaign rally. “Thousands of people were cheering.”

The following day, Trump reiterated his lie, claiming that the supposed celebration “was well covered at the time.”

“There were people that were cheering on the other side of New Jersey, where you have large Arab populations. They were cheering as the World Trade Center came down,” Trump said on ABC’s “This Week.”

The lie, which has been roundly debunked, appeared to originate from an article published a few days after the attacks, reporting that law enforcement officials had investigated “a number of people who were allegedly seen celebrating the attacks and holding tailgate-style parties on rooftops while they watched the devastation on the other side of the river.”

But Trump, as he frequently does, exaggerated the allegation in the story ― an allegation that was never substantiated.

Following the rally in which he first peddled the lie, Trump mocked one of the journalists who wrote the original story, veteran New York Times reporter Serge Kovaleski, after Kovaleski himself affirmed that the story did not back up Trump’s lie.

“I certainly do not remember anyone saying that thousands or even hundreds of people were celebrating,” Kovaleski said.

In response, Trump performed a disgusting imitation of Kovaleski, who has arthrogryposis, a congenital joint condition.

On the actual day of the attacks, Trump, a New York real estate mogul, tastelessly bragged about his downtown Manhattan building, 40 Wall Street.

Calling in to a New York TV news broadcast, as the station aired footage of the World Trade Center towers collapsing, Trump claimed that his property would now become the tallest building in the area.

“40 Wall Street actually was the second-tallest building in downtown Manhattan, and it was actually, before the World Trade Center, was the tallest — and then, when they built the World Trade Center, it became known as the second-tallest,” he said. “And now it’s the tallest.”

Trump interview on 9/11: "[My building] was the 2nd-tallest in Manhattan... And now it’s the tallest." #NeverForget pic.twitter.com/47WUsSKLRu — Ess (@ScottyLiterati) September 11, 2016

That claim also turned out to be false.

Trump has frequently referred to the attacks on Twitter, his favorite medium of communication.

In 2011, he claimed that he’d foreseen the attacks.

I predicted the 9/11 attack on America in my book "The America We Deserve" and the collapse of Iraq in @TimeToGetTough. — Donald J. Trump (@realDonaldTrump) December 29, 2011

And on the anniversary of the attacks in 2013, he tweeted his “best wishes to all, even the haters and losers, on this special date.”

“@realDonaldTrump: I would like to extend my best wishes to all, even the haters and losers, on this special date, September 11th.” — Donald J. Trump (@realDonaldTrump) September 12, 2013

This article has been updated to include Trump’s remarks at the Pentagon ceremony.