Facebook on Monday sued DLA Piper and three other law firms alleging they “knew or should have known” that the claim made by their client, Paul Ceglia, that he owned half of Facebook was a fraud.

Ceglia sued Facebook founder and CEO Mark Zuckerberg in 2012, alleging Ceglia had invested $1,000 in Facebook in 2003 and Zuckerberg promised to give him 50 percent of the company in return.