LONDON (AP) — A 57-year-old man was arrested Tuesday for allegedly possessing tear gas that is believed to have caused the partial evacuation of a terminal at Heathrow Airport.

About 20 people treated by paramedics were not seriously injured in the incident that closed the departures area of Terminal 4 for about three hours late Monday, the Metropolitan police said.

It was not believed to be terror-related, police said, but the investigation is ongoing.

Police, firefighters and ambulance crews responded to a potential hazardous material incident. Officers found a canister that was believed to contain CS gas, a synthetic irritant used by police as tear gas

The man was held in custody after being arrested on suspicion of possessing CS spray, which is considered a firearm in Britain. He also was arrested on suspicion of causing a public nuisance.

Suspects in England are not named unless they are charged with a crime.