Oct. 7, 2009 (United Press International) -- Canadian authorities and Imperial Oil (AMEX:IMO) (TSX:IMO) Wednesday were trying to learn what caused a leak of benzene in Sarnia, Ontario, a day earlier.

The incident happened about 1 p.m. at the Imperial processing facility in southwestern Ontario and prompted an emergency three-hour lockdown of the neighborhood around it, the Sarnia Observer reported.

Benzene is used to manufacture gasoline and is a known carcinogen.

Imperial Oil spokeswoman Julie Ferguson said it wasn't immediately known how much of the chemical escaped or why, but it was contained quickly. She said heavy foam was sprayed on the spill to prevent vapors from spreading.

Area residents were told to remain indoors with all windows closed, and students downwind were kept inside for three hours, the newspaper said.

No injuries were reported.

There was a similar spill from the same tank on March 14, 2008, which Imperial officials said was caused by the collapse of its floating roof, the Observer said.