A transportation agency plans to file a lawsuit Monday alleging that United Airlines is falsely claiming to buy huge amounts of jet fuel out of a small, rural Illinois office that doesn't even have a computer to avoid paying tens of millions of dollars in taxes in Chicago, where the purchases are allegedly being made.

The Regional Transportation Authority alleges United Aviation Fuels Corp., a subsidy of United Airlines, has operated a "sham" office in the DeKalb County community of Sycamore since 2001 after reaching an agreement to pay the town more than $300,000 a year -- a fraction of what it would have owed in sales taxes in Chicago and Cook County.