ARLINGTON, Va. (WUSA) -- The Arlington County Commonwealth's Attorney and the Arlington County Police Department have opened an investigation against a U.S. Congressman's son, police said in a news release.

Patrick Moran, son and former Director of Field Operations for his father Jim Moran (D-VA), is being investigated for "assisting another to vote illegally," police said in a statement.

This investigation was sparked after a video showed Patrick Moran allegedly committing voter fraud.

Patrick Moran resigned from his father's campaign after a conservative group released an undercover video in which he discussed a plan to cast fraudulent ballots.

The resignation came hours after Project Veritas, an organization led by activist James O'Keefe, released a video showing an undercover operative pitching a voter-fraud plan to Patrick Moran outside an Arlington restaurant earlier this month. The plan called for casting ballots in the name of 100 voters who were registered but rarely voted.

In the video, Patrick Moran expresses doubts about the plan but eventually tells the undercover volunteer to "look into it."

Patrick Moran replied to a 9News e-mail asking for comment saying:

"In reference to the "O'Keefe" video, at no point have I, or will I ever endorse any sort of illegal or unethical behavior. At no point did I take this person seriously. He was acting crazy and it struck me that he was joking, and for only that reason did I humor him.

In retrospect, I should just walked away, making it clear that there is no place in the electoral process for even the suggestion of illegal behavior, joking or not.

In regards to my position on the campaign, I have stepped down because I do not want to be a distraction during this year's critical election. "

Jim Moran's campaign released this statement following Pat's resignation:

"Patrick is well liked and was a well-respected member of the campaign team. This incident, however, was clearly an error in judgment. The campaign has accepted Patrick's resignation, effective immediately."

Brian Moran, Jim Moran's brother and Chairman of Virginia's Democratic Party, released this statement:

"Patrick Moran is my nephew and a good kid who in this instance, made a mistake. He has taken responsibility and has stepped down from the Moran campaign. The Democratic Party of Virginia is an organization that stands for free and open participation in our elections and we oppose even the appearance of any effort to undermine those values."

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