As part of a larger crackdown on pollution from coal-fired power plants, the U.S. Department of Justice and the state of Illinois sued an Edison International (EIX) unit Thursday for alleged Clean Air Act violations at six Illinois coal-fired power plants.

Federal and state officials allege that the plants, owned by Edison unit Midwest Generation, have been illegally emitting "massive amounts" of pollutants including nitrogen oxides, sulfur dioxide and soot for years, particularly after they were modified in the 1990s by their previous owner, Exelon Corp. (EXC) unit Commonwealth Edison Co., without required pollution-control equipment.

Regulators are asking the court to order Midwest Generation to install new pollution-control technologies at the plants. They are also seeking civil fines for the alleged violations.

A U.S. Justice Department spokesman declined to comment on the total amount of fines that federal and state regulators are seeking.

The lawsuit potentially could expose the company to tens of millions of dollars or more in fines and perhaps hundreds of millions in pollution-control costs.

Regulators allege that the Clean Air Act violations have occurred for a decade, and maximum fines for such violations range from $27,500 to $37,500 per day.

The U.S. EPA notified Midwest Generation in 2007 that the plants were violating the Clean Air Act.

The Natural Resources Defense Council and other environmental groups notified Midwest Generation in late July that they planned to sue the company over Clean Air Act violations cited in "numerous" EPA notices. The groups cited a study from the Harvard School of Public Health that found that pollution from nine Chicago-area coal plants were responsible for 311 premature deaths a year, as well as 4,100 emergency-room visits and 21,500 asthma attacks annually.

Midwest Gen spokesman Douglas McFarlan said the company believed an agreement it reached with Illinois in 2006 over emissions at its Illinois coal plants was sufficient, and that additional pollution control was unnecessary.

However, that agreement was narrowly focused on ensuring the company could comply with the state's mercury-emissions rules and had nothing to do with federal and state pollution requirements with which Midwest Generation is required to comply, said Illinois Attorney General Lisa Madigan.

"Today's lawsuit alleges that for many years Midwest Gen has and continues to violate both federal Clean Air Act and Illinois Environmental Protection Act provisions that limit emissions of particulate, sulfur dioxide and nitrogen oxides - emissions that aggravate asthma, cause acid rain and increase ozone levels," Madigan said in an email.

Midwest Gen is open to exploring a possible settlement, McFarlan said.

The Justice Department lawsuit is part of a national initiative to "stop illegal pollution from coal-fired power plants," the agency said in a statement. In addition to Midwest Gen, the agency has sued three other coal-fired power-plant operators this year for Clean Air Act violations: Westar Energy Inc. (WR) and NRG Energy Inc. (NRG) unit Louisiana Generating LLC.

In 2005 Dynegy Inc. (DYN) settled a lawsuit filed by the EPA, the state of Illinois and others over Clean Air Act violations at three Illinois coal plants. The company agreed to spend about $545 million to install pollution control equipment at the plants, as well as pay a $9 million civil penalty and spend $15 million on environmental projects.

Edison International, based in Rosemead, Calif., owns California utility Southern California Edison.

Edison International shares closed down 1.2% at $33.66.

-By Cassandra Sweet and Brent Kendall, Dow Jones Newswires; 415-439-6468; cassandra.sweet@dowjones.com