Defense lawyers for the man convicted of killing congressional intern Chandra Levy filed court papers Tuesday asking for a new trial, claiming federal prosecutors knew—or should have known—a key witness gave false testimony.

A District of Columbia Superior Court jury found Ingmar Guandique guilty in 2010. At trial, one of Guandique’s former cellmates, Armando Morales, testified that Guandique confessed to the killing. Morales told the jury he had never come forward to law enforcement before, but Guandique’s lawyers contend that was a lie.