GRAND JUNCTION, Colo. — State regulators have ruled out three energy companies as the source of benzene that contaminated a western Colorado spring and sickened a man.

The Colorado Oil and Gas Conservation Commission said Thursday it's still investigating whether a fourth company, Williams Production RMT, is the source.

A Williams spokeswoman says the company doesn't believe it's the source but is cooperating with investigators.

Ned Prather of De Beque was hospitalized after drinking water from a spring at his cabin in May 2008. Tests found the spring contained benzene, a cancer-causing substance.

State investigators say they have ruled out Marathon Oil Co., Petroleum Development Corp. and Nonsuch Natural Gas as possible sources.