(Reuters) An Illinois mass transit agency on Monday sued United Airlines, saying the world’s No. 1 air carrier is operating a “sham” sales office in a smaller city to evade paying millions of dollars in taxes in Chicago, where it is based.

The Regional Transportation Authority, which oversees train, bus and commuter rail service in the Chicago area, said United was improperly seeking to dodge higher sales taxes by “accepting” jet fuel at an office set up in Sycamore, Illinois, where the combined tax rate is 8 percent. That compares with a rate of 9.5 percent in Chicago.

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