Several major solar panel companies have been issued subpoenas as part of a federal investigation into whether they overcharged for their work in order to squeeze more stimulus money out of taxpayers. 

The investigation was revealed earlier this year by one of the subjects in the probe, California-based SolarCity. Shortly before going public last week, the company disclosed in an SEC statement that it "and other companies with significant market share" -- as well as others "related to the solar industry" -- had received subpoenas from the Treasury Department's Office of the Inspector General. 

The SEC statement did not reveal what other companies are being scrutinized. But The Washington Post reported that SunRun and Sungevity have also received Treasury subpoenas. 

The investigation follows the controversies over Solyndra, the California solar firm that declared bankruptcy in September 2011 after pocketing nearly $530 million in taxpayer support, and over other government-backed solar companies that have gone belly up. 

In this case, the Treasury Department IG apparently is looking into whether companies that likewise received hundreds of millions of dollars charged more than they should have for work on rooftop solar panels. 

SolarCity said it was not aware of any "specific allegations of misconduct" but revealed that the investigation is looking at "possible misrepresentations concerning the fair market value" of solar power systems eligible for grant money. 

"SolarCity has followed the Treasury Department’s guidance for the 1603 program and has done nothing wrong," spokesman Jonathan Bass told "We are sharing information as part of the government review and are happy to do so."

Together, the three companies reportedly have claimed more than $500 million in taxpayer support. Bass confirmed that SolarCity has applied for $341 million in grants.

At issue is a Treasury-administered aid fund rooted in the stimulus bill called the 1603 program. 

As of July, the program had doled out $13 billion to roughly 45,000 projects across the country. 

The program offers cash payments to alternative energy companies worth 30 percent of any given project's cost. The money is available for everything from solar to wind to geothermal to fuel cell projects. 

Representatives with SunRun and Sungevity did not return requests for comment Tuesday from 

A spokesman with the Treasury Department IG's office declined to comment on the specifics of the case, or confirm whether SunRun, Sungevity and others were being looked at. 

"We can generally say that we are carrying out our Inspector General Act-mandated duties to monitor the process by which public funds are distributed, to be sure that they are granted properly and used properly, consistent with applicable law and intended use," Richard Delmar said in a statement. 

He said that, as a "high-impact" program involving "a significant amount of public funds," the 1603 program has a "high priority in our audit and investigative plans." 

The companies have political ties and are run in some cases by prominent political contributors. SolarCity is backed by Elon Musk, a prolific contributor to Democrats and Republicans who is also behind electric car company Tesla and space company SpaceX. Employees from all three companies donated to President Obama in the 2012 cycle.