Facebook attorneys are calling an upstate man’s federal lawsuit claiming part ownership of the company “a fraud on the court.”

In their latest legal response, Facebook attorneys accuse Paul Ceglia of doctoring a 2003 contract that he says proves he bought into Facebook founder Mark Zuckerberg’s idea for the site when Zuckerberg was a Harvard freshman.

Ceglia “has now come out of the woodwork seeking billions in damages,” the response filed Thursday in US District Court in Buffalo said.

Ceglia’s lawsuit, first brought in 2010 and refiled last month, relies largely on a two-page “work for hire” contract bearing the names of both men. Ceglia says he and Zuckerberg signed the contract after Zuckerberg responded to his Craigslist help-wanted ad for work on a street-mapping database.

According to the lawsuit, Ceglia paid Zuckerberg $1,000 to develop software for the street-mapping project, and gave him another $1,000 after Zuckerberg told him about his Facebook idea, with the condition Ceglia would get half if it took off.