At least 40 people were injured, some seriously, after a man accidentally dropped a tear gas device in a police station in the Bulgarian capital.
Initial reports suggested the gas was a type of pesticide
The man was in a visitors' area in a Sofia traffic police office when the "unpremeditated accident" happened, the Interior Ministry said.
At least four people, including a police officer, were reported to be seriously ill from gas inhalation.
The man, described as a 51-year-old butcher, was later arrested.
The device, described as a self-defence teargas spray, was "freely available in shops", Interior Ministry official Boiko Borissov told AFP news agency.
The incident occurred at 1000 (0700GMT), when the office was crowded with people.
The building was evacuated as firemen and civil defence specialists arrived.
Officials later said the gas content in the air had returned to safe levels.
"We have measured for poisonous emissions in the air... and there is no further danger to the public," a civil defence official said.
The health ministry first identified the chemical as chloropicrin, a pesticide that has also been used in the past as a military-grade tear gas.
It can be extremely dangerous when inhaled in large quantities.