TUCSON, Ariz. — Tucson fire officials are investigating what caused an overnight building explosion that injured five people.

Firefighters say several 911 calls reported smoke and flames late Wednesday night at an oil processing facility.

Tucson Fire Department spokesman Andy Skaggs says firefighters also had to deal with a partial building collapse and a broken gas line.

Five employees were taken to the hospital. Authorities say two were later flown to Phoenix for extensive injuries.

A sixth worker did not need treatment.

The building is occupied by Desert Whale Jojoba Company. According to its website, the company produces and supplies raw materials for cosmetics.

Officials say it took nearly 50 firefighters to get the blaze under control.

An estimate of damages was not immediately known.