Subsequent testing confirmed that a powdery substance discovered by the Secret Service at the White House on Sunday was cocaine, a law enforcement official said Wednesday, as questions linger about how the drug got into the building.

The discovery of the substance caused a brief shutdown of the White House after it was found by Secret Service officers in a common storage area 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 in a zip-close bag near an entrance where visitors taking tours are directed to leave their phones, the official said.

White House press secretary Karine Jean-Pierre said President Biden has been briefed on the incident and noted the area where the cocaine was discovered is “heavily traveled” by visitors.

“When it comes to visitors to the West Wing, they come for many reasons, obviously we do have West Wing tours that occur here on campus,” she said, adding tours took place Friday, Saturday and Sunday.

The Secret Service is not ruling out any White House personnel, guests or visitors. 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.

White House staffers frequently give guests tours of the West Wing, often at night and over the weekend. Guests must pass through security before entering the White House complex and then are asked to leave phones in small cubbies.

An initial test of the substance conducted by the District of Columbia Fire Department soon after it was found indicated it was cocaine, and the additional test took place Tuesday night.

In a recording of a radio message from D.C. Fire hazmat personnel who responded to the scene Sunday night, a first responder can be heard saying the substance tested positive for “cocaine hydrochloride.” The message was posted by the website OpenMHz, which records and archives radio dispatches by police, fire and EMS agencies.

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. 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.

Unlike some other federal entities, the Secret Services does not have drug-sniffing dogs. If an officer encounters a powder or substance on White House grounds or in surrounding areas, they routinely request the assistance of D.C. Fire and EMS.

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

Mr. Biden was not at the White House over the weekend. He, first lady Dr. Jill Biden and members of their family departed for Camp David on Friday. They returned to the White House on Tuesday for an event with the National Education Association and Fourth of July festivities.