Federal authorities are investigating whether former Representative George Santos engaged in insider trading by betting on a prediction market about whether he would show up at President Trump’s State of the Union address in late February.

Just before the speech, Mr. Santos announced on social media that he planned to attend. Whether he would be there or not was a hot topic among online bettors on the prediction market Kalshi, who were wagering on the guest list.

“I’m gonna be in the gallery,” Mr. Santos teased in a video on X.

But Mr. Santos missed the speech, and around the time of the event, Kalshi detected that Mr. Santos had bet against his own attendance, according to a person familiar with the matter who spoke on condition of anonymity to discuss an ongoing investigation.

The company referred the matter to the Justice Department and the Commodity Futures Trading Commission, a financial regulator that oversees prediction markets, the person said.

The C.F.T.C. is now investigating Mr. Santos, according to another person, who also spoke on condition of anonymity. It was not immediately clear whether the Justice Department had opened an investigation.

Mr. Santos did not immediately respond to a request for comment. A spokeswoman for Kalshi declined to comment.

The investigation comes as the Trump administration is under pressure to show it can police insider trading and other abuses on the fast-growing, highly lucrative prediction markets. Kalshi is one of several prediction market companies with ties to the president’s business empire. Early last year, the firm appointed Donald Trump Jr., the president’s oldest son, as a “strategic advisor.”

A New York Times investigation published last month found that under the Trump administration, the C.F.T.C., a little-known but crucial financial regulator, has repeatedly ruled in favor of prediction markets. Two senior career officials at the agency who raised questions about the handling of prediction market cases were put on investigative leave, barred from the office and placed under investigation late last year, The Times reported.

In an interview with The Times, Michael S. Selig, the chairman of the C.F.T.C., promised that regulators would hold wrongdoers to account.

A former Republican congressman from New York, Mr. Santos, 37, was charged with fraud in 2023, after The Times and other outlets reported that he had lied extensively about his biography. Prosecutors accused him of lying on official forms and stealing from donors, among other schemes. He was expelled from the House and ultimately sentenced to seven years in prison.

Mr. Santos was released from prison in the fall after Mr. Trump commuted his sentence.

The investigation into Mr. Santos’ prediction market trading was reported earlier by National Public Radio.

The inquiry is the third insider trading case to come to light in recent weeks. Last month, the Justice Department and the C.F.T.C. charged a Google employee with using insider information to bet on internet search results.

And in April, a member of U.S. Special Forces was charged with illegally using confidential government information to make more than $400,000 betting on the operation to capture Nicolas Maduro, the president of Venezuela.

Benjamin Weiser, Glenn Thrush and Jonah E. Bromwich contributed reporting.