WHITING, Ind. (AP) — Federal regulators say BP PLC violated the Clean Air Act by beginning to make modifications at its Indiana oil refinery along Lake Michigan to process Canadian crude before it received the proper permit.

The allegation announced Thursday is included in an amended complaint by the U.S. Environmental Protection Agency. The complaint alleges BP violated the law by making several unapproved changes in 2005 when it altered a unit at the Whiting refinery that converts heavy oils into lighter products such as gasoline.

The EPA said in November the modifications caused "significant increases" in sulfur dioxide, nitrogen oxide, particulate matter and carbon monoxide emitted from the refinery.

The Indiana Department of Environmental Management approved a construction permit in May for BP to begin a $3.8 billion expansion at the refinery, the nation's fourth-largest, about 20 miles southeast of downtown Chicago. Environmental groups are challenging the IDEM permit, saying state regulators improperly granted it.

BP spokesman Scott Dean said in a written response to the EPA complaint that BP has cooperated with the agency since November "and will continue to meet with the agency to resolve this difference of opinion regarding interpretation of the rules. We are confident these preliminary findings will prove to be baseless. In terms of actual emissions, the Whiting Refinery has reduced its air emissions by 68 percent since 2001."

He said the issue centers on whether routine safety and reliability improvements to equipment that does not significantly affect air emissions requires an environmental permit from EPA or triggers new source rules.

"The work performed in 2005 did not require a permit according to the analysis performed BP. The 2005 project was evaluated and designed as a free-standing project years before BP began the Canadian heavy crude project," he said.

George Czerniak, chief of the EPA's air-enforcement branch in Chicago, said the complaint is based on the record of activities at the refinery.

BP has said the expanded refinery would be the nation's top processor of heavy high-sulfur Canadian crude oil, boosting its annual production of gasoline, diesel fuel and jet fuel by 15 percent to about 4.7 billion gallons annually.

The company has 30 days to request a conference with the EPA. Czerniak said BP can provide the EPA any information it wants the agency to consider as it moves forward.

BP could provide information that convinces EPA officials no violations occurred, he said. If they can't, the EPA would try to work with BP to resolve the complaint. The agreement could call for additional pollution controls and could involve penalties, Czerniak said.

If no agreement is reached, the case could go before a federal judge, he said.