WASHINGTON — An unapologetic Energy Secretary Steven Chu defended a half-billion-dollar federal loan to a solar-panel manufacturer that went belly up, even as he told a House committee Thursday he was unaware of dozens of key details that led to the debacle over Solyndra.

Under hours of hostile questioning from Republicans on the House Energy and Commerce Committee, Chu declined several opportunities to say he was sorry, but acknowledged that in hindsight the deal was “extremely unfortunate” and “regrettable.”

“Certainly knowing what I know now, we’d say ‘no,’” Chu said during a daylong hearing before the energy panel’s subcommittee on investigations. “But you don’t make decisions fast-forwarding two years in the future and then go back. I wish I could do that.”

Chu insisted that politics played no role in his department’s decision to loan Solyndra $528 million before it went bankrupt and laid off 1,100 workers.

Testifying under oath on a widening controversy, Chu took responsibility for the disastrous 2009 loan, but said he was unaware of many details about the loan or financial problems that Solyndra faced — including predictions by Energy Department staff two years ago that the company was likely to face severe cash-flow problems.

Chu repeatedly said he didn’t know until recently of problems with Solyndra or suggestions of political interference on the company’s behalf by the White House or Energy Department officials.