According to a report by The Huffington Post, Mr Harr set off its newly installed metal detectors as he sought to enter Congress on Thursday, prompting an official to scan him with a hand wand that revealed he was carrying a firearm under his suit jacket.
The Maryland representative had been intending to vote on a waiver to allow retired US Army general Lloyd Austin to serve as president Joe Biden’s new defence secretary.
Blocked from entering after a discussion with security personnel, Mr Harris was then seen talking to New York GOP colleague John Katko, who reportedly declined to look after the weapon for him.
Instead, Mr Harris disappeared before returning 10 minutes later, leaving his keys and smartphone with officials and passing through the magnetometers as normal before going on to cast his vote.
“One security official present saw a firearm on the person of Rep Harris and relayed that to his superiors,” a Capitol official told NBC News. “To be clear, Rep Harris did not enter the floor.”
“USCP is investigating the matter,” a spokesman for the police force said of the incident, declining to comment further.
The metal detectors were installed under orders from House speaker Nancy Pelosi in the wake of the Capitol riot on 6 January, when a violent mob of Donald Trump supporters storm the building to protest his election defeat, breaking into offices and engaging in looting and vandalism.
The melee left five people dead and saw the since-departed president impeached by the House for an unprecedented second time.
But the enhanced security measures have been opposed by Republican populists like Lauren Boebert and Madison Cawthorn, who have boasted about flouting DC laws to openly carry guns in the city for protection in the wake of the failed insurrection.
Speaker Pelosi has since proposed that any failure to comply with security searches will lead to fines of $5,000 for a first offence and $10,000 for a second, although these rules are not yet in force.
Ms Boebert, along with GOP peers Andy Biggs and Rick Allen, also reportedly refused to submit to searches by metal detection wand on Thursday after setting off the magnetometers.