OXFORD, Miss. -- Investigators haven't found any ricin in the house of Mississippi man accused of mailing poisoned letters to President Barack Obama, a U.S. senator and a local judge, according to testimony Monday from an FBI agent.

Agent Brandon Grant said that a search of Paul Kevin Curtis' house in Corinth, Miss., on Friday did not turn up ricin or ingredients for the poison. A search of Curtis' computers has found no evidence so far that he researched making ricin.

The detention and preliminary hearing began Friday in U.S. District Court in Oxford, Miss., but was continued when it ran into the evening.