A Lyons cement plant failed to install the best pollution controls while tainting Boulder County’s air for possibly more than a decade, according to a lawsuit filed Tuesday by the U.S. Environmental Protection Agency.

In the action filed in U.S. District Court in Denver, the EPA alleges that Cemex Inc., which produces Portland cement at its Lyons facility, committed several federal violations since early 1997 after it made plant modifications to improve production. The EPA says those improvements led to violations of the Clean Air Act, and it’s asking the court to assess civil penalties that could reach into the billions of dollars.

David Rochlin, an attorney for the EPA, said any court-approved fine would likely be substantially smaller for logistical reasons.

Still, he said, the breadth of the 20-page suit and the potentially large penalties helped make this a "major case."

"This is one of our bigger cases, and we hope we can work with the company to negotiate some sort of settlement that includes conforming to the Clean Air Act and some financial penalties," Rochlin said.

The plant improvements at the heart of the suit were made before Cemex purchased the plant from Southdown Inc. in 2000. A Cemex spokeswoman said Tuesday that those improvements didn’t lead to the violations alleged by the EPA and that the "allegations simply are not supported by the facts."

"We dispute the allegations and are very disappointed, particularly in today’s economic climate, that the government has chosen a path of litigation rather than working with the company to resolve its concerns," Jennifer H. Borgen, Cemex’s director of external communications, said in a statement. "Over the last three years, Cemex has voluntarily made more than $4 million worth of environmental improvements at the Lyons plant alone."

The lawsuit is the latest in a string of government actions taken against the plant. The EPA put the company on notice of the violations in 2007, and the state of Colorado cited Cemex for environmental infractions last summer.

In the suit filed Tuesday, the most serious alleged violations are "significant" increases in nitrous oxide emissions. Those were caused by the plant modifications, according to the EPA, and required the company to obtain a special operating permit.