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Voting by illegal immigrants is easy to do and is being done in the United States, a former ICE agent explained on "The O'Reilly Factor."

President-elect Donald Trump made headlines over the weekend with a tweet about his defeat to Hillary Clinton in the popular vote.

In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally — Donald J. Trump (@realDonaldTrump) November 27, 2016

Many on the left accused Trump of spreading an outright lie about illegal immigrant voters.

To assess the claim, O'Reilly talked to retired ICE Special-Agent-In-Charge Claude Arnold, who said he "routinely" arrested illegal immigrants who had voter registrations.

"I worked in six locations across the United States. I probably arrested more than 1,000 illegal aliens in my career and I routinely encountered people who were in possession of voter registration cards. And as a part of my interview of them when they were arrested, I would ask them, "Do you or have you voted?' and often, I would get the answer 'yes,'" he explained.

Arnold agreed with O'Reilly's statement that "it's easy to do it and it's being done."

Arnold said that there is a very high demand for counterfeit documents like a drivers license because a large swath of the illegal population want to work.

He said there are often several illicit document vendors in neighborhoods with high illegal immigrant populations.

Because some states only require a drivers license number to be recorded to register to vote, or apply for an absentee ballot, the illegal registrations "are not necessarily picked up," he said.

Watch the full interview above.

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