Authorities are investigating death threats against Rep. Markwayne Mullin (R-Okla.), according to an aide to the freshman congressman.

Mullin’s chief of staff, Karl Ahlgren, said that Mullin and his family are being protected, the Associated Press reported.

Ahlgren said his office would have no further comment, according to the AP.

U.S. Capitol Police Officer Shennell Antrobus told the news service that the agency is investigating, but would not specify the threat.

Mullin, the owner of a plumbing business, won the open race to replace Rep. Dan Boren (D-Okla.), a Blue Dog Democrat who chose to retire.

Earlier this month, Mullin joined House Republicans who warned the Obama administration they would oppose any attempt to bypass Congress on the issue of gun control by issuing a White House executive order that limits access to guns.

Mullin accused the administration of trying to usurp the ability of Congress to write the nation's laws.

"Under the U.S. Constitution, Congress is required to be the primary source of federal legislation, and I will fight to ensure that Congress does not abandon our responsibility to the American people by delegating our role in government to the executive branch, particularly regarding such an important issue as the Second Amendment," he said.