Republicans are prodding the Treasury Department to issue a long-overdue investigation into the abuse of clean energy tax credits by renewable energy companies, suspecting that agency resistance to the investigation is to blame for the delay.

Sens. Lisa Murkowski of Alaska and Jeff Flake of Arizona sent a letter to the Treasury's inspector general Monday, asking that the investigation into the abuses of solar energy subsidies be issued swiftly.

"As you are aware, the department recently indicated that applicants included ineligible costs or otherwise overstated the value of their solar energy investments by claiming approximately $1.3 billion in unwarranted cash grants: more than two-and-a-half times the amount of the Solyndra default," the letter reads.

The Solyndra default is a notorious case of a solar company going bankrupt after receiving a federal loan guarantee worth $535 million.

The senators want the complete details of the inspector's investigation, including the total value of tax credit claims going back more than a decade, noting a letter from the Treasury Department that says resistance from other agencies is to be expected. The department says resistance would likely result in delays in getting the results of the investigation to Congress.

"We hope such interference will not cause any unnecessary delays in the case," the senators say.

They want a response from Treasury by Earth Day, April 22.

The tax credits in question include the section 48 and 1603 programs, which were created under President Obama's 2009 economic stimulus program.

The tax credits were meant to create increased flexibility for solar and other renewable energy projects to receive the subsidy.

"Given that the department has acknowledged that the review process for this program 'is no different than what might take place upon examination by the IRS if the applicant elected the Section 48 tax credit rather than the 1603 payment,' the magnitude of such disclosures could prove more egregious when the same scrutiny is applied to the closely related solar investment tax credit," the letter says.