The New York Times has slammed Republican presidential candidate Donald Trump after he appeared to ridicule a reporter with a congenital joint condition that limits movement in his arms.

At a rally in South Carolina on Tuesday, Trump defended his widely discredited claim that thousands of Muslims in New Jersey cheered as the World Trade Center collapsed on Sept. 11, 2001. He then appeared to impersonate reporter Serge Kovaleski, one of the authors of a 2001 article in The Washington Post that referred to “a number of people who were allegedly seen celebrating the attacks.”

“Now, the poor guy — you ought to see the guy: ‘Uh, I don’t know what I said. I don’t remember,'” Trump said, as he contorted his arms in an apparent imitation of Kovaleski, who suffers from arthrogryposis.

Kovaleski told MSNBC on Monday that he did “not recall anyone saying there were thousands, or even hundreds, of people celebrating.” He made similar comments to CNN.

“We think it’s outrageous that he would ridicule the appearance of one of our reporters,” a spokesman for The Times told NBC News. Kovaleski is currently an investigative reporter for The Times.

Kovaleski hit back at Trump, too. “The sad part about it is,” he told The Washington Post this week, “it didn’t in the slight bit jar or surprise me that Donald Trump would do something this low-rent, given his track record.”

Trump on Thursday claimed he was not familiar with Kovaleski’s appearance and not aware of his condition.

“I have no idea who this reporter, Serge Kovalski (sic) is, what he looks like or his level of intelligence. I don’t know if he is J.J. Watt or Muhammad Ali in his prime – or somebody of less athletic or physical ability,” Trump said in a statement released to NBC News. “If Mr. Kovaleski is handicapped, I would not know because I do not know what he looks like.”

Trump claimed that he had “merely mimicked what I thought would be a flustered reporter trying to get out of a statement he made long ago.” He said he has “tremendous respect for people who are physically challenged.” But he added that he thinks Kovaleski “should not have tried to pull back from his statement. He should live with it and as a professional stand by what he said.”

The Washington Post article cited by Trump, dated Sept. 18, 2001, included the following paragraph:

In Jersey City, within hours of two jetliners’ plowing into the World Trade Center, law enforcement authorities detained and questioned 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.

Trump has said the article is “evidence” that “thousands and thousands” of Muslims cheered the fall of the Twin Towers.

Officials have said that did not happen, and The Washington Post has added a disclaimer to the online edition of the article.