SUPERIOR, Wis. — The Environmental Protection Agency says it has finished monitoring the air at the site of a recent oil refinery explosion in northwestern Wisconsin and found no elevated levels of anything toxic.

The blast at the Husky Energy refinery in Superior last Thursday injured at least 13 people, caused a widespread evacuation and sent billowing plumes of black smoke into the air.

WCCO-TV reports EPA officials say the agency will continue to provide oversight for additional monitoring done by a third-party company, GHD.

The U.S. Chemical Safety Board says the initial explosion took place in the fluid catalytic cracking unit. Wisconsin Public Radio reports the board will study why the metal failed in the unit that exploded.

Local authorities continue to work with federal agencies to determine what caused the explosion.