First the White House, now Congress — or close to Congress, anyway.

Capitol Police said Wednesday the agency was investigating how a small bag that tested positive for cocaine wound up in its headquarters, located about a 10-minute walk from where the House and Senate meet.

The one-inch by one-inch zip-locking baggie containing a white powder was found on a hallway floor near offices that deal with Crime Scene, Intel and Reports Processing, officials said in a statement.

“The hallway is on the second floor in an area that has been a staging spot for furniture and supplies,” USCP helpfully added.

“The area is heavily trafficked by various contractors and employees.”

The bag was found just before 1 p.m. Wednesday by an officer and field tested positive for the illegal drug, according to USCP.

Capitol Police’s Investigations Division is investigating the matter, including searching for DNA evidence that could sniff out a culprit.

USCP headquarters is located on D Street Northeast, on the same block as two Senate office buildings and blocks from Union Station.

The latest drug discovery came just over 10 months after cocaine mysteriously appeared at the White House.

On July 2, Secret Service agents discovered a small baggie of white powder in a West Wing storage locker, feet away from the Situation Room and one floor below the Oval Office.

That discovery prompted an 11-day investigation that the Secret Service wrapped up without naming a suspect.

In closing the case, the protective agency cited “a lack of physical evidence” after tests yielded no fingerprints and insufficient DNA.

Because so much sensitive material passes through the area, no cameras are focused there — and were thus unable to pick up footage of the culprit depositing the white, powdery substance in one of the 182 lockers in a vestibule, a source familiar with the investigation told The Post at the time.