President Trump expressed support Tuesday for the mother of a police officer killed in a fatal car crash involving an illegal immigrant, telling her "we're with you" after she said there was a quiet mass-murder happening in the U.S.

"This is actually a mass murder happening in the United States by illegal aliens killing American citizens. Over 63,000 American citizens have been killed since 9/11 by illegal aliens. It's a crime spree that is being left unchecked," Mary Ann Mendoza said during a roundtable discussion at the White House.

Mendoza's son, Sgt. Brendan Mendoza of the Mesa Police Department in Arizona, was killed in 2014 by an intoxicated man driving the wrong way down the street.

"The sanctuary city officials — state and city officials — they are putting American lives at harm every single day. We have become collateral damages to their personal agendas," Mendoza said during the roundtable event, which was called to denounce local policies of noncooperation with federal immigration authorities.

"We're with you. I never heard the figure 63,000 people killed by illegal immigrants, is that a known figure?" Trump asked Mendoza, who said the number was extrapolated from an average rate.

It was not immediately clear where Mendoza located the statistics, but a 2011 Government Accountability Office report said that a study population of 249,000 criminal aliens had been arrested for 25,064 homicides.

"Sixty-three thousand people since 9/11. Boy, that is some number, I've never heard that number before. It's an incredible number," Trump said.