A small amount of a white powdery substance was found in the White House on Sunday evening, according to a person familiar with the episode, and an initial test by emergency response workers determined that it was cocaine.

President Biden and his family were away at Camp David when a uniformed Secret Service agent found the substance during a patrol of the West Wing around 6 p.m. on Sunday, the person said, prompting a brief shutdown of some of the White House campus as response workers assessed whether the substance was hazardous.

Later that evening, a test of the substance by hazmat crew officials returned a result: “We have a yellow bar saying cocaine hydrochloride,” an official said on a dispatch sent at 8:49 p.m. and logged by a website that gathers the agency’s radio communications.

A person familiar with the investigation, who spoke on the condition of anonymity to discuss an open inquiry, said that the radio dispatch discussing the test of the substance was placed between the District of Columbia Fire Department’s hazmat team and other officials within the department.

Anthony Guglielmi, a spokesman for the Secret Service, said that the substance had been found in a “work area of the West Wing” and that another dispatch that said the substance had been found in a White House library was incorrect.

In a statement, Mr. Guglielmi said “the item was sent for further evaluation and an investigation into the cause and manner of how it entered the White House is pending.”

He did not confirm or dispute the initial finding that the powder was cocaine.

Vito Maggiolo, a spokesman for the District of Columbia Fire Department, declined to confirm the contents of any of the dispatches, but said officials with the department “were requested to assist the Secret Service as they conducted an investigation.”

On Tuesday morning, Mr. Biden returned to the White House with several of his family members.