Three U.S. solar panel firms are being investigated to determine if they inflated costs to get government payments, The Washington Post reported Friday.

The Treasury Department's office of inspector general has subpoenaed the financial records of SolarCity, SunRun and Sungevity to determine if they qualified for more than $500 million in federal grants and tax credits they received for performing work, sources familiar with the probe told the Post.

The solar companies received the money through President Obama's stimulus initiative to offer cash grants to clean-energy developers.

The three firms, working mostly in the sun-rich states of California and Arizona, collected hundreds of millions of dollars in federal grants covering a share of their costs on thousands of home installations during the past three years.

But the companies allegedly charged higher prices for their work than is normal in the solar installation industry, solar experts and Treasure investigators said.

SolarCity spokesman Jonathan Bass said Thursday the company believes its estimates were fair and its costs in line with guidelines issued by the Treasury Department at the time.

Sungevity and SunRun declined to comment, the Post said.