Investigators have not found ricin in the home of a man accused of mailing poison-laced letters to President Barack Obama and two other officials.
FBI Agent Brandon Grant testified in federal court that a search of Paul Kevin Curtis' house in Corinth, Mississippi, did not turn up ricin or ingredients for the poison.
He said: "There was no apparent ricin, castor beans or any material there that could be used for the manufacturing, like a blender or something." The FBI agent speculated that Curtis could have thrown away the processor.
Curtis' lawyer said her client denies any involvement in sending the ricin-laced letters, which were mailed last week to Mr Obama, US Senator Roger Wicker and a Lee County, Mississippi, judge.
Christi McCoy said in court that someone may have framed her client, suggesting that a former co-worker with whom Curtis had an extended exchange of angry emails may have set him up.
Still, Mr Grant testified that authorities believe that they have the right suspect.
Federal investigators believe the letters were mailed by Curtis, an Elvis impersonator who family members say suffers from bipolar disorder.
The letters to Mr Obama and Mr Wicker were intercepted at mail sorting facilities before they arrived at the US Capitol building and the White House.
Grant testified Friday that authorities tried to track down the sender of the letters by using a list of Sen Wicker's constituents with the initials KC, the same initials in the letters.
He 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, Tennessee, postmark, which includes many places in north Mississippi.
Wicker's staff recognised Curtis' name as someone who had written to the senator before, Mr Grant said.
The letters also contained lines that were on Curtis' Facebook page, including the phrase: "I am KC and I approve this message", Mr Grant added.
All the envelopes and stamps were self-adhesive, meaning they will not yield DNA evidence. Mr Grant said thus far the envelopes and letters have not yielded any fingerprints.
Ms McCoy said the evidence linking the 45-year-old to the crime has hinged on his writings posted online, which were accessible to anyone.
Much of Monday's testimony focused on Curtis' prior run-ins with police and evidence about his mental health.
"The fact that this man may be suffering from a form of mental illness, how does this make it make it more likely than not that Mr Curtis committed to these crimes?" Ms McCoy asked.
Mr Grant said that it didn't, but said past evidence about mental state, "helps establish a potential behaviour background for Mr Curtis, perhaps not realising what he's doing".