The discovery of the substance caused a brief shutdown of the White House after it was found by Secret Service officers on the ground floor of the West Wing, which houses the Oval Office and offices of some of the president’s top aides and support staff. It was found near an entrance where visitors taking tours are directed to leave their phones, the official said.

The Secret Service is not ruling out any White House personnel, guests or visitors. But at this point, one leading theory is that the bag was brought in by an individual on a White House tour of the West Wing, according to senior law enforcement officials briefed on the matter.

The Secret Service is leading a full review of how the substance got into the West Wing, law enforcement officials said, including examining cameras and entrance logs to determine who had access to the space.

Authorities are still working to identify the person who left the cocaine at the White House. Officials caution this will be a challenging investigation, and while there are some cameras in the West Wing, it’s unclear if anyone was captured on those cameras with the bag of cocaine.

The discovery of the “unknown item” prompted a “precautionary closure” of the White House, Secret Service spokesman Anthony Guglielmi acknowledged in a statement Tuesday.

Guglielmi said that the D.C. Fire Department was called to evaluate the substance and determined it was “non-hazardous.”

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