SUPERIOR (AP) -- 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.

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

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