Secret Service agents found the powder Sunday during a routine sweep of the White House; it was in a small, clear plastic bag on the floor in a heavily trafficked area, according to three people, who were not authorized to speak about the ongoing investigation and spoke to the Associated Press on condition of anonymity.

On Wednesday, Press Secretary Karine Jean-Pierre said the White House had confidence in the Secret Service.

“The president thinks it’s incredibly important to get to the bottom of this,” she said.

Biden was at Camp David with members of his family for the long Fourth of July holiday weekend when the powder was discovered and the complex was briefly evacuated as a precaution.

Officials immediately suspected the powder was an illicit drug because of how it was packaged, two of the unnamed sources said. The fire department was called in to determine whether the substance was hazardous, and its initial test was positive for cocaine. A more sensitive lab analysis confirmed the results on Wednesday.

Investigators have not determined who brought the drug into the White House. The Secret Service, which is responsible for securing the complex, is combing through visitor logs and security footage.

Many staffers and official visitors enter the White House through the lobby where the drug was found. It is also open on weekends and evenings for invitation-only West Wing tours led by White House staff for their friends, family and other guests. Staff members can request evening or weekend tour slots, but there is often a long wait list.

If a White House employee brought in the drug, it should be easier to determine, because staff are fingerprinted and subjected to drug tests. A visitor would be harder to pin down; tours of the West Wing were given on Friday, Saturday and Sunday.

The Secret Service said in a statement Tuesday that the White House was closed as a precaution, and that “an investigation into the cause and manner of how it entered the White House is pending.”