The investigation centers on a video of Villanueva dressed in civilian clothing and asking for support. In the video, the sheriff speaks into the camera and says the message is aimed at deputies, whom he asks to consider making donations to his campaign.
In a statement Wednesday, Tiffany Blacknell, director of communications for the District Attorney's office, confirmed the investigation.
"This video has come to our attention. We take these matters seriously and it is currently under review. A formal investigation by our PID unit has been initiated. We cannot comment further at this time."
In the video, the sheriff said the Association for Los Angeles Deputy Sheriffs -- the union representing deputies -- was not funding his reelection campaign. ALADS, which pumped money into the sheriff's original campaign four years ago, endorsed Villanueva this year, but he said the group wasn't contributing financially.
"It's up to each and every one of you who you want to be as sheriff," he said on the video. "We're gonna win this thing, and Lord willing, and if you want to help, anything will help us get our message out there, get our ads online and on TV and our texting going on."
He concludes by asking for any type of donation to help his campaign.
According to The Los Angeles Times, state law and sheriff's department policy both prohibit an agency employee from soliciting political contributions from other employees.
In a statement, Villanueva's campaign team said they have "worked diligently to run an effective campaign in compliance with all applicable laws. To date, we have no official communication from the DA. We will work with the DA's office to address any concerns they may have."
LA County sheriff's race: Luna has big lead but Villanueva not ready to concede
Semi-official results from Tuesday's election show Villanueva trailing in his reelection bid against former Long Beach police Chief Robert Luna. Thus far, Luna had about 57% of the vote to Villanueva's 43%. The race likely won't be decided, however, until all outstanding ballots are tallied. It was unclear exactly how many ballots are still remaining.
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City News Service contributed to this report.