From a Washington Post story by Peter Hermann and Tyler Pager headlined “Secret Service investigating suspected cocaine found in White House”:

A preliminary test indicated that the white powder found inside the White House Sunday evening, prompting a brief evacuation, was cocaine, according to two officials familiar with the matter and the recording of a dispatch from a D.C. fire crew that responded to the incident.

A spokesman for the Secret Service, Anthony Guglielmi, said the substance is undergoing further testing to determine what it is, and authorities are looking into how it got into the White House. He said the D.C. fire department determined the substance was found in a “work area of the West Wing.”