NASHVILLE, Tenn. (AP) — A clean-energy advocacy group called for an investigation Tuesday after finding that the Tennessee Valley Authority bought two corporate jets, a luxury helicopter and another plane in recent years. The utility says its inspector general is already reviewing the matter.

The Southern Alliance for Clean Energy focused on the 2015 and 2017 model jets, 2013 helicopter with a Mercedes-Benz upgrade trim and 2015 turboprop airplane.

TVA says the aircraft were bought for about $35.3 million.

TVA maintains that the planes and helicopters are necessary to meet customers and stakeholders and pursue economic development by attracting companies to the region.

Alliance executive director Stephen Smith called the aircraft purchases “the very definition of corruption of the TVA mission.” The clean energy group did not specify who should investigate, but called for oversight from TVA’s inspector general and Congress.

“Buying extravagant jets and helicopters is a hijacking of the TVA Act, whose stated purpose is to protect the residential customers, not buy flashy toys for millionaire executives or cut backroom deals with private industry,” Smith said in a news release.

TVA executive vice president of operations Mike Skaggs said the inspector general review of its aircraft is underway. Other electric utilities similar to TVA have the same type of equipment, Skaggs added.

“In most cases, the planes that others use that are our peers, are bigger and more expensive than the ones we use,” Skaggs said.

TVA spokesman Jim Hopson said private aircraft are the only safe, timely option across TVA’s 80,000-square-mile, seven-state service area, which largely lacks commercial flights.

Operating the modern jets costs just 7 percent more than the older turboprops, with improved safety and performance, Hopson said.

TVA services Tennessee and parts of Georgia, Alabama, Mississippi, Kentucky, North Carolina and Virginia.

The aircraft purchases drew criticism from others besides clean energy advocates.

Debbie Dooley, who helped found the ongoing national Tea Party movement and is president of Conservatives for Energy Freedom, said the purchases show TVA CEO Bill Johnson needs to be replaced.

Johnson, the highest-paid federal employee in the nation, receives a compensation package of more than $6 million, including retirement and other benefits. The TVA board has said Johnson’s pay is still low compared to salaries of utility executives not in public service.

Elder Jimmie Garland, vice president of the Tennessee State Conference of the NAACP, said the findings show the need for independent oversight of the nation’s largest public utility.

Skaggs said TVA’s fleet includes the two jets, a dozen helicopters of different types and the turboprop plane, which is no longer being flown and is waiting to be sold.

Other TVA helicopters support inspection, maintenance and repairs of TVA’s 16,000-mile transmission system.

TVA bought the used luxury helicopter for almost $7 million in 2015, Skaggs said.

Since 2013, Skaggs said TVA has helped attract more than 330,000 jobs to its region and almost $40 billion worth of investment. He said the Mercedes-Benz helicopter helps TVA give companies a quick flyover and explanation of the electric rates when they are considering moving to the area.

“The cost of the helicopter is great. It is a big expenditure.” Skaggs said. “But it’s a great capital investment.”