Nerve agent alarm prompts evacuation of Dirksen Federal Courthouse, but no substance found

A nerve agent alarm was tripped Friday at the Dirksen Federal Courthouse, prompting an evacuation and an hour-long investigation by the Chicago Fire Department that determined the building was safe.

The scene was secured at 1:22 p.m. and people returned to the affected areas, according to fire department spokesman Larry Merritt.

The alarm went off around noon, indicating a possible harmful gas between the 2nd and 16th floors of the courthouse, 219 S. Dearborn St., Merritt said.

Crews evacuated those floors and raised the alarm to a “HazMat level 2,” the highest level possible.

“Crews monitored around the sensor and its intake, but found no evidence of a substance,” Merritt said. There were no injuries or transports, and the alarm was reset.

It’s unclear what set off the alarm.

In February 2019, fire crews responded to a false alarm for a chemical spill at the courthouse. The building’s alarms indicated a hazardous substance in the air, but firefighters’ equipment showed normal air quality readings inside the building.

Nerve agents are man-made, highly poisonous gases that attack the body’s nervous system. In 1995, terrorists used the nerve agent sarin in an attack at the Tokyo subway system, killing 13 people and injuring more than 5,000.