The U.S. Capitol Police department has launched an investigation after Rep. Andy Harris (R-Md.) set off a magnetometer near the House chamber while carrying a concealed gun Thursday afternoon, according to a Hill staffer with knowledge of the incident.

Harris set off the magnetometer — installed after the deadly Jan. 6 riot at the Capitol — as he was on his way to the chamber, leading security to pat him down, said the staffer, who spoke on the condition of anonymity because he was not authorized to speak about the incident.

“One security official present saw a firearm on the person of Rep. Harris and relayed that to his superiors,” the staffer said in a text message. “To be clear, Harris did not enter the Floor.”

Eva Malecki, a spokeswoman for the Capitol Police, said the agency is “investigating the matter” but declined to elaborate, saying she could not discuss an ongoing investigation.

A spokesman for Harris, 63, a six-term congressman who lives in Baltimore County, did not answer questions about the incident. He instead offered a statement that Harris’s chief of staff, Bryan Shuy, sent Fox Baltimore on Thursday night:

“Because his and his family’s lives have been threatened by someone who has been released awaiting trial, for security reasons, the Congressman never confirms whether he nor anyone else he’s with are carrying a firearm for self-defense,” Shuy wrote. “As a matter of public record, he has a Maryland Handgun Permit. And the congressman always complies with the House metal detectors and wanding. The Congressman has never carried a firearm on the House floor.”

Members of the public are not allowed to carry guns on Capitol grounds. But members of Congress may keep firearms in their offices or transport them on the Capitol grounds if they are unloaded and securely wrapped. Lawmakers are not allowed to bring guns into either the House or Senate chambers.

The Post obtained hours of video footage, some exclusively, and placed it within a digital 3-D model of the building. (TWP)

It is unclear why Harris had the gun with him Thursday, two weeks after a violent mob stormed past law enforcement and into the building, injuring dozens and leading to the death of a police officer and four rioters.

A HuffPost congressional reporter who witnessed part of the incident, Matt Fuller, reported that Harris lingered near the elevators after security turned him away. He then tried to ask another lawmaker, Rep. John Katko (R-N.Y.) to take the gun for him.

Katko refused, Fuller reported, saying he did not have “a license.” Harris left the area and returned several minutes later, successfully going through the magnetometer and proceeding onto the House floor. Katko’s office did not respond to requests for comment.

On Friday, Yvette Lewis, chair of the Maryland Democratic Party, called on Harris to resign, calling him “an embarrassment to our state and to the good people of the first district.”

Former Maryland state delegate Heather Mizeur said on Facebook that Harris’s behavior on Thursday and since the Capitol siege “has stirred something deep within me” — leading her to think seriously about challenging Harris in 2022.

Mizeur, who unsuccessfully sought the 2014 Democratic gubernatorial nomination, would face an uphill climb in Harris’s solidly red district. She said she first started thinking about running when he objected to Biden’s electoral victory and nearly got into a physical confrontation with another lawmaker during the debate on the electoral college vote.

She also criticized Harris for saying he would seek a seventh term, despite pledging a decade ago not to stay in office for more than six terms.

The gun incident, Mizeur said, was “the tipping point.”

Harris, the only Republican in the Maryland congressional delegation, took office in 2011 representing Maryland’s Eastern Shore as well as parts of Carroll, Harford and Baltimore counties. A staunch ally of former president Donald Trump, the practicing anesthesiologist parroted Trump’s false claims of election fraud. He is a member of the House’s most conservative group, the House Freedom Caucus, and has been a tenacious opponent of D.C. autonomy, marijuana legalization, pandemic restrictions and stay-home orders.

Harris’s gun incident is the latest to heap scrutiny on lawmakers who may be carrying weapons at the Capitol. Some Republicans have expressed outrage about the installation of the metal detectors in the wake of the riot and in a few cases have blown through them and refused to cooperate with searches after setting them off.

Freshman Rep. Lauren Boebert (R-Colo.), who has pledged to carry her Glock in Washington and around Congress, reportedly would not let officers search her bag after setting off the metal detector Jan. 12.

House Speaker Nancy Pelosi (D-Calif.) said Friday that when the House is back in session next month, they will vote to fine lawmakers $5,000 if they refuse to cooperate with a metal-detector screening. A second offense could lead to a $10,000 fine. “It is sad that this step is necessary, but the disrespectful and dangerous refusal of some Republican Members to adhere to basic safety precautions for our Congressional Community — including our Capitol Police — is unacceptable,” Pelosi wrote in a memo to colleagues.