"The searches are concluded, not one single shred of evidence was found to indicate Kevin could have done this," McCoy told reporters after the hearing Monday.
McCoy also questioned why Curtis would have signed the letters "I am KC and I approve this message," a phrase he had used on his Facebook page.
McCoy said in court that someone may have framed Curtis, suggesting that a former business associate of Curtis' brother, a man with whom Curtis had an extended exchange of angry emails, may have set him up.
Still, Grant testified that authorities believe they have the right suspect.
Grant said lab analysis shows the poison in the letters was in a crude form that could have been created by grinding castor beans in a food processor or coffee grinder.
Grant testified Friday that authorities tried to track down the sender of the letters by using a list of Wicker's constituents with the initials KC, the same initials in the letters. Grant said the list was whittled from thousands to about 100 when investigators isolated the ones who lived in an area that would have a Memphis, Tenn., postmark, which includes many places in north Mississippi. He said Wicker's staff recognized Curtis as someone who had written the senator before.
All the envelopes and stamps were self-adhesive, Grant said Monday, meaning they won't yield DNA evidence. He said thus far the envelopes and letters haven't yielded any fingerprints.
Associated Press writer Holbrook Mohr contributed to this report from Jackson, Miss.