The chief executives of the biggest U.S. health insurance companies were called to testify about industry practices before a congressional hearing examining coverage, costs and claim denials.

U.S. Representative Dennis Kucinich sent letters dated Wednesday to Aetna Inc (AET.N), Cigna Corp (CI.N), Humana Inc (HUM.N), UnitedHealth Group Inc (UNH.N) and WellPoint Inc (WLP.N), among others.

The letter asks the companies' CEOs to testify at a Sept. 17 hearing about the nature, cost/benefit, and impact of administrative measures and protocols used by the health insurance industry to determine coverage.

Kucinich, a Democrat, chairs the domestic policy subcommittee of the Oversight and Government Reform Committee in the House of Representatives.

A copy of the letter sent to Aetna CEO Ronald Williams was obtained by Reuters. A congressional source told Reuters similar letters were sent to the other CEOs.

Spokespeople for Aetna and Wellpoint said the companies were reviewing the letter. A UnitedHealth spokesman confirmed the company had received the letter. Officials at other insurers could not immediately be reached.

Last week, other lawmakers in the House and Senate sent requests to major health insurers seeking information about industry practices.

The requests come amid a tumultuous debate over larger efforts to reform the U.S. healthcare system, with Democrats pushing for more competition and access to affordable health insurance plans, among other changes. (Reporting by Kim Dixon and Lisa Richwine; editing by Andre Grenon and Carol Bishopric)