FREMONT, Calif. - FBI agents executed search warrants yesterday at the headquarters of solar panel maker Solyndra Inc., which received more than $500 million in federal loans before seeking bankruptcy protection last week.

An FBI spokeswoman said she could not provide details.

Solyndra spokesman Dave Miller said agents were collecting documents but the company did not know the reason for the search.

The assumption was that the search was related to the loans, he said. Those loans have for months been the subject of a probe by the House Energy and Commerce Committee.

“The FBI raid further underscores that Solyndra was a bad bet from the beginning and put taxpayers at unnecessary risk,’’ representatives Fred Upton, Republican of Michigan and chairman of the energy committee, and Cliff Stearns, a Florida Republican who chairs the House Oversight and Investigations Subcommittee, said in a statement.

The raid came about a week after Solyndra said it was laying off 1,100 workers.

Like other solar energy companies, it faced declining prices, in part because of heavy competition from Chinese companies.

The company had been held up as a model for government investment in green technology. President Obama visited Solyndra last year, saying it represented the future of American renewable energy innovation.

Solyndra’s technology relied on a solar tube that could soak up sunlight from many angles, producing energy more efficiently and using less space. But other companies caught up and provided similar products at a lower cost.

“It’s really sad to everybody who worked here,’’ Miller said. “We believed in the technology.’’

Obama administration officials have said that the $535 million loan guarantee -$528 million was actually lent - was also sought by the Bush administration. Solyndra increased sales revenue by 2,000 percent in the past three years, and private investors put more than $1 billion into the company.

“The Department of Energy’s Loan Program has supported a robust, diverse portfolio of more than 40 projects that are investing in pioneering companies as we work to regain American leadership in the global race for clean energy jobs,’’ the department said.

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