The building at 26 Federal Plaza in Lower Manhattan was evacuated on Thursday afternoon following reports of an unknown white powder.

The FDNY said a call came in just before 4 p.m. saying the powder was reported found in the mailroom on the ninth floor of the building, which is home to the Department of Homeland Security and the New York City field office of U.S. Immigration and Customs Enforcement, among other agencies. First responders and hazmat units were on the scene with four minutes, authorities said.

"Today's event was under control almost immediately, without incident," Deputy Mayor for Public Safety Kaz Daughtry said.

5 letters containing powder were found, FBI says

At an evening news conference, authorities said they are awaiting test results on the powder, which was in five separate letters, adding those letters would also be sent to FBI headquarters in Quantico, Virginia.

Officials said at least two people were exposed to the powder.

Christopher Raia, the assistant director of the FBI's New York City field office, said, "Weapons of Mass Destruction and Hazard Evidence response teams immediately responded to the scene. The building was evacuated as per standard protocol and right now no threat remains to any employees or the public at this time."

DHS said in a statement, in part, that "some individuals were sheltering in place as part of established safety protocols."

Officials are asking people to avoid the area as they continue to investigate.

Debate continues over what's going on at 26 Federal Plaza

The building at 26 Federal Plaza has been the scene of numerous detainments by ICE agents following routine immigration hearings. It has also been the site of several protests, alleging immigrants are being held in deplorable conditions.

DHS maintains the building is not a detention center and says allegations of overcrowding or poor conditions are "categorically false."

"It is a processing center where illegal aliens are briefly processed to be transferred to an ICE detention facility," DHS Assistant Secretary Tricia McLaughlin said in a recent statement to CBS News New York. "All detainees are provided with proper meals, medical treatment, and have opportunities to communicate with their family members and lawyers. As we arrest and remove criminal illegal aliens and public safety threats from the U.S., ICE has worked diligently to obtain greater necessary detention space while avoiding overcrowding."

A federal judge on Tuesday said he would block the Trump administration from using the building to hold immigrants facing deportation unless it reduces the number of detainees and improves conditions at the site, including by providing sleeping mats and hygiene products.

U.S. District Judge Lewis Kaplan issued the temporary restraining order after holding a hearing earlier Tuesday. A government lawyer conceded in court that those detained by ICE at the facility did not have access to certain services, including sleeping mats, in-person legal visits, medication and more than two meals per day.