Secretary of State Mike Pompeo is facing accusations of a Hatch Act violation after being featured as a speaker at the Republican National Convention on Tuesday night.

Rep. Joaquin Castro, D-Texas, who chairs the House Foreign Affairs Subcommittee on Oversight and Investigations, demanded answers from Pompeo's Deputy Secretary Stephen Biegun in a letter on Tuesday.

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"It is highly unusual, and likely unprecedented, for a sitting Secretary of State to speak at a partisan convention for either of the political parties," Castro wrote. "It appears that it may also be illegal."

Castro pointed out that Biegun told State Department employees in February that he would be "sitting on the sidelines of the political process this year" because he is a Senate-confirmed official like Pompeo.

U.S. Secretary of State Mike Pompeo and Israeli Prime Minister Benjamin Netanyahu make joint statements to the press after their meeting, in Jerusalem, Monday, Aug. 24, 2020. (Debbie Hill/Pool via AP)

The Hatch Act is a federal law that limits certain political activities of federal employees.

Pompeo delivered a pre-taped speech from Jerusalem to the Republican National Convention on Tuesday that praised President Trump’s foreign policy agenda while avoiding stepping into election politics by criticizing Democratic presidential nominee Joe Biden.

Castro's colleague Rep. Raja Krishnamoorthi, D-Ill., called for Office of Special Counsel to look into the potential Hatch Act violations.

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"As a Member of Congress who believes the Hatch Act should be enforced at the White House, and everywhere else it applies, the Special Counsel needs to address what we’ve been witnessing this week from the Trump Administration," Krishnamoorthi wrote on Twitter on Wednesday.

House Minority Leader Kevin McCarthy responded Wednesday by saying that President Obama's Health and Human Services Secretary Kathleen Sebelius was found to have violated the Hatch Act.

"Mike Pompeo ... used no government funds," McCarthy told "Mornings with Maria." "At any time we hold our convention, there's a very good chance you're going to find Mike Pompeo somewhere else around this world. If that's the only argument you have back to the Republican Convention because you can't compete on the ideas, that shows the weakness in the Democratic Party."

Rep. Castro's brother, former Housing and Urban Development Secretary Julian Castro, was also found to have broken the Hatch Act during a 2016 Yahoo News interview.

The Office of Special Counsel determined that "Secretary Castro's statements during the interview impermissibly mixed his personal political views with official agency business despite his efforts to clarify that some answers were being given in his personal capacity."

Fox News' inquiry to the State Department was not immediately returned.

Fox News' Andrew O'Reilly and Brooke Singman contributed to this report.