Fulcrum BioEnergy, a clean-fuels pioneer that raised more than $1 billion to turn household waste into lower-emitting fuels for planes and trucks, is in danger of going under.

The company recently laid off nearly all of its staff of about 100 and halted most of its operations, according to more than a half-dozen former employees. It signals the apparent demise of a company that garnered funding from a litany of industry giants — including BP, United Airlines, Cathay Pacific and Japan Airlines — and a setback in the push for a clean-fuels breakthrough to lower emissions from the aviation industry.

Fulcrum employees said they were laid off on May 17 and the company’s website stopped working last week. It’s unclear which Fulcrum executives are still employed, but several didn’t respond to emails and phone messages seeking comment. Nuveen, a primary financial backer of the company, declined to comment.