LOS ANGELES — A toxic plume of chromium has spread to multiple groundwater wells near the California desert town made famous by the movie "Erin Brockovich."

In a letter sent Monday to Pacific Gas & Electric, which had discharged the chromium decades ago, water regulators said wells near Hinkley are showing increased concentrations of the cancer-causing substance despite efforts to contain it.

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"These concentrations exceed background concentrations and show an increasing trend over time," the letter from the Lahontan Regional Water Quality Control Board stated.

The board on Monday also ordered the utility to conduct more tests after discovering more polluted wells.

"In parts of the chromium plume, concentrations of total chromium exceed the current drinking water standard ... thereby rendering areas of the groundwater unusable," the regulators added.

A company spokesman said PG&E is complying with the directive.

PG&E tried to stop the spread after the contamination was exposed during a 1996 case in which the utility settled with Hinkley residents for $333 million.

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PG&E thought the contamination was contained after tests in 2008 showed the plume was spreading.

In 2008, PG&E agreed to pay $20 million to settle the last in a series of lawsuits over the wells.

The agreement involved claims that 104 people were exposed to water that contained chromium 6, a possible carcinogen.

The lawsuits claimed PG&E contamination sickened hundreds of people in Kings, Riverside and San Bernardino counties from the 1950s through the mid-1980s.

In 2006, PG&E agreed to pay $295 million to settle other lawsuits involving about 1,100 people.

Erin Brockovich, then a law firm assistant, rallied Hinkley's 600 residents to sue PG&E. That battle was the basis for the 2000 movie staring Julia Roberts.

The Associated Press contributed to this report..