Federal law enforcement officials raided three central Illinois facilities of manufacturer Caterpillar on Thursday as part of an investigation the company said may be related to business with its Swiss subsidiary CSARL.

Officials from three federal agencies, some wearing jackets that said "police, federal agent", searched the Peoria, Illinois, headquarters of Caterpillar, one of the world's largest makers of construction and other heavy equipment. Facilities in East Peoria and Morton, Illinois, also were raided under a federal warrant, U.S. Attorney's office spokeswoman Sharon Paul said.

"We believe the execution of this search warrant is regarding, among other things, export filings that relate to the CSARL matter," Caterpillar said in a statement later on Thursday, referring to its Swiss subsidiary.

Caterpillar spokeswoman Corrie Heck Scott said in an email that the company was cooperating with law enforcement.

Paul said the agencies involved included the Internal Revenue Service's criminal investigation unit, the U.S. Department of Commerce Office of Export Enforcement and the Federal Deposit Insurance Corp.'s office of inspector general. She declined to comment further on the details of the investigation.

The exact reason for the raid was unclear, but Caterpillar told the Securities and Exchange Commission in a filing last month that the IRS had notified the company it owed $2 billion more in taxes for the years 2010 to 2012 because of profits from its Swiss unit. Caterpillar said it would "vigorously contest" the tax bill.

"We believe that the relevant transactions complied with applicable tax laws and did not violate judicial doctrines," Caterpillar said.

Two years ago the SEC notified Caterpillar that it was conducting an "informal investigation" relating to CSARL and asked the company to preserve relevant documents.

A 2014 report by U.S. Senate Democratic staff said that Caterpillar had avoided paying $2.4 billion in U.S. taxes since 2000 by shifting profits to the affiliate in Switzerland.

The report said Caterpillar paid PricewaterhouseCoopers $55 million to develop the tax strategy. Under the strategy, Caterpillar transferred the rights to profits from its parts business to a wholly controlled Swiss affiliate called CSARL, even though no employees or business activities were moved to Switzerland, the report said.

In exchange, CSARL paid a small royalty, and the income was taxed at a special rate of 4 to 6 percent that Caterpillar negotiated with the Swiss government, the report said.