Grant also testified that Curtis' ex-wife said Curtis once told her that he was in hostage situation in Chicago after a breaking up with a former girlfriend, threatened suicide and shot a gun in the air.
However, the agent said they haven't been able to find a record of that.
Grant's testimony ended Friday evening, but the hearing is set to continue Monday morning.
In court documents, Curtis' attorney, Christi McCoy, gave some details of Curtis' arrest. Curtis had gone to get his mail outside his home and was planning to go to his ex-wife's home to cook dinner for her and their children when he was approached by officers in SWAT gear, she wrote. He was then interrogated at an FBI office for several hours, handcuffed and chained to a chair.
Curtis cooperated to the best of his ability, but when he suggested he might need a lawyer, an agent discouraged that, McCoy wrote.
According to an FBI affidavit, the letters he sent read: "Maybe I have your attention now even if that means someone must die."
Officials have confirmed that the letters contained ricin.
While the toxin can be extremely lethal in its purest form, experts say more crude forms are relatively easy to make.
The FBI has not yet revealed details about how the ricin was made or how lethal it may have been. It was in a powdered form inside the envelopes, but the FBI said no one has been sickened by it so far. A senate official said Thursday that the ricin was not weaponized, meaning it wasn't in a form that could easily enter the body.
More than a dozen officials, some wearing hazardous materials suits, were searching the home Friday where Curtis was arrested in Corinth, Miss. FBI spokeswoman Deborah Madden would not say if authorities have found ricin or materials used to make it in Curtis' home, and officials have not provided details about how Curtis may have either obtained or made the ricin.
Curtis' ex-wife has said he likely didn't have the know-how to make ricin, and she did not know where he would buy it because he was on disability. But Cohen said ricin was once known as "the poor man's bioterrorism" because the seeds are easy to obtain and the extraction process is relatively simple.
"Any kid that made it through high school science lab is more than equipped to successfully make a poison out of this stuff. Any fool can get recipes off the Internet and figure out how to do it," Cohen said.
Those seeds, which look a bit like coffee beans, are easy to buy online and are grown around the world; they are often used to make medicinal castor oil, among other things. However, using the seeds to make a highly concentrated form of ricin would require laboratory equipment and expertise to extract, said Raymond Zilinskas, a chemical and biological weapons expert.
"It's an elaborate process," he said.
Gresko reported from Washington. Associated Press Photographer Rogelio Solis in Corinth and writers Emily Wagster Pettus in Jackson, Jay Reeves in Oxford and Eric Tucker in Washington contributed to this report.
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