OXFORD (AP) — An FBI agent has testified that he could not say whether agents have found ricin in the home of a Mississippi man accused of mailing the toxin to the president and a senator.

Agent Brandon M. Grant testified at a hearing Friday in Oxford for Paul Kevin Curtis. Testimony ended Friday evening, but the hearing was to continue Monday morning.

Grant stressed that the investigation is ongoing and that he does not know what may have been found with the hearing approaching two hours long. He says investigators searching the house did find a package they were interested in. But he did not yet know what was in it.

Grant says investigators are looking at the possibility that others were involved, but he says Curtis is the primary suspect.

The ricin mailed to the president and a U.S. senator is relatively easy to make but generally can’t be used to target a large number of people, experts say.

A Mississippi man, Curtis, 45, has been charged with mailing letters laced with the naturally occurring toxin to President Obama and U.S. Sen. Roger Wicker. Authorities say he sent a third threatening letter to a Mississippi judge, though that letter is still being tested for the presence of ricin. Curtis has denied making the ricin and mailing the letters.

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.

Law enforcement agents should be able to test the toxin found in the letters to determine its potency and purity, as well as learn what chemicals may have been used to extract it from widely available castor beans, said Murray Cohen, the founder of the Atlanta-based Frontline Foundation, which trains workers on preparedness and response to bioterrorism and epidemics. Those chemicals might then be able to be linked to purchases made by Curtis or materials found in his home.

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.

Cohen said ricin is not common because other poisons, such as anti-freeze, can easily be bought at a store. And it’s not a weapon of choice for mass casualties because it would need to be eaten or inhaled to be most deadly.

“You can put this stuff in an envelope, but how are you going to get the intended person to inhale or ingest it?” Cohen said.

Authorities say Curtis sent a letter that may have contained ricin to Sadie Holland, a judge who sentenced him to six months in jail in an assault case a decade ago. Holland’s son, Democratic Rep. Steve Holland, said Friday that his 80-year-old mother has undergone medical tests and had no signs of poisoning. He said she had done a “smell test” of the threatening letter, telling him it burned her nose a bit.

If swallowed, the poison can in a matter of days shut down the liver and other organs, resulting in death, according to the federal Centers for Disease Control and Prevention. If inhaled, it can cause respiratory failure, among other symptoms. No antidote exists.

The most notable case of ricin poisoning was in 1978, when a Bulgarian dissident was lethally injected with ricin by an operative of that country’s secret service.

Meanwhile, Curtis appeared in federal court Friday afternoon for a hearing in Oxford. He was ushered into the courtroom in an orange jail jumpsuit and shackles. He turned to face a young woman in the audience before the hearing and whispered, “I didn’t do it.”

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.

Under questioning by Curtis’ attorney, FBI Agent Brandon M. Grant testified Friday that he could not say whether ricin had been found in Curtis’ home and stressed that he did not know what may have been found as the hearing approached two hours. He did say that investigators found a package they were interested in, but he did not know what was in it.

Grant also testified that there was one fingerprint on the letter sent to the judge, but that it didn’t match Curtis. He said several people handled the letter, and DNA and other tests are pending.

Grant said authorities were still trying to determine whether there were any co-conspirators, but Curtis is the main focus.

Grant’s testimony ended Friday evening, but the hearing is set to continue Monday morning.

McCoy said the only evidence linking her client to the mailings are postings online. She said after the hearing Friday that she has seen no hard scientific evidence thus far. The federal government has offered no evidence he had ricin or castor beans in his possession, she said.

Curtis’ brother Jack Curtis and his daughter, 20-year-old Madison, said afterward they also were not convinced by the government’s case.

“After hearing what I heard in this courtroom, it appears to me that the reason I haven’t been provided any evidence is there appears to be none,” Jack Curtis said.

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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.