An investigation is underway into who left a chemical mixture in the witness waiting area of Philadelphia’s Southwest Detectives Division on Friday night, triggering coughing and eye irritation among employees and an evacuation of the building.

A preliminary inspection by the Philadelphia Fire Department, the Department of Licenses and Inspections, the Philadelphia Office of Emergency Management, the Police Department’s Command Inspections Bureau, and the Department of Homeland Security has concluded the irritant was a combination of Mace and an unknown substance, police said Saturday.

The outbreak began around 9:15 p.m. Friday. Police said there were no witnesses in the waiting area of the district headquarters at 5510 Pine St. and no suspects.

Licenses and Inspections completed cleaning the building and declared it safe to return to normal operations just before 2 a.m., police said.

Homeland Security is handling the investigation.