WASHINGTON – A formal laboratory test of the white matter found at the White House on Sunday came back positive for cocaine, according to an official with knowledge of the investigation.

The substance was found Sunday night in a small zippered bag in a busy part of the West Wing and prompted a brief evacuation, according to a US Secret Service official.

It is not clear how long the bag was in the White House. The blurred timeline and the number of people walking through the area where the cocaine was found could make it difficult to determine who was responsible for the substance, a source familiar with the investigation told NBC News on Tuesday.

The Secret Service is conducting the investigation, which will include looking at cameras and entry logs, according to a source familiar with the investigation.

When asked about the incident on Wednesday, White House press secretary Karine Jean-Pierre stressed that the investigation is under the control of the Secret Service and that she is «confident» they will get to the bottom of the situation.

The White House «is subject to rigorous guidelines including drug testing, so we will take whatever action is appropriate and warranted, pending the outcome of the Secret Service,» Jean-Pierre told reporters, adding that he would not «get into hypotheticals».

She said President Joe Biden was briefed on the situation, even though he and his family were at Camp David, not the White House, when the cocaine was discovered.

“The president thinks it’s very important to get to the bottom of this,” Jean-Pierre said in response to a question about how determined Biden is to find out who brought drugs into the White House.

Jean-Pierre reiterated that the cocaine was found in a busy area that visitors often pass through.

Senator Tom Cotton, Republican of Arkansas, sent a letter to Secret Service Director Kimberly Cheatle on Wednesday, asking her to release information about where the cocaine was found. She also asked half a dozen questions about access to the White House, other cases of illegal drugs found in the complex and security procedures, requesting that her questions be answered by July 14.

«If the White House complex is not secure, Congress needs to know the details, as well as your plan to fix any security flaws,» Cotton said.

The cocaine was found by officers during «routine patrols» of the White House, a Secret Service official said.

The substance had tested positive for cocaine in a preliminary field test, a source familiar with the investigation said Tuesday, and the formal laboratory test confirmed that result.

Peter Alexander, Monica Alba, Sally Broston, mike memoli and frank thorp contributed.