Several other solar companies that received federal loan guarantees are now facing scrutiny. (Creative Commons)

Two Solyndra executives will refuse to answer questions at a congressional hearing Friday, their attorneys wrote in a letter to a House subcommittee on Tuesday.

Solyndra, a manufacturer of solar panels, declared bankruptcy earlier this month despite amassing more than $520 million in federal loans. The implosion has been a destructive stain for President Barack Obama's clean energy investment program.

The FBI, the Department of Energy and the House Energy and Commerce Committee's Subcommittee on Oversight and Investigations are all investigating the $30 billion federal loan program.