Linqto, the once-highflying private stock investment platform, has filed for bankruptcy protection, citing investigations into its business and questions about what its customers even own.

Linqto was one of the first to capitalize on small investors’ hunger for the hottest private companies, including Elon Musk’s SpaceX and artificial-intelligence company Anthropic, and it became a test case for the risks to individual investors entering the lightly regulated market for pre-IPO shares. The platform grew to maintain about $500 million on behalf of thousands of investors around the world.