Records seized by the FBI also showed that Tupelo martial arts instructor Everett Dutschke ordered castor bean seeds, used to make ricin, from eBay, FBI Special Agent Stephen Thomason said in an eight-page affidavit.
Dutschke, who is being held in jail without bond, is expected to appear in U.S. district court in Oxford, Mississippi, for a detention hearing on Thursday.
Contacted by Reuters, Dutschke's attorney, public defender George Lucas, declined to comment. In several media interviews before his arrest Dutschke maintained his innocence.
Soon after the seeds were delivered to Dutschke's home address, someone using his laptop computer downloaded a publication on safe handling and storage of the poison, Thomason stated.
An agent last week retrieved the contaminated dusk mask from a trash bin near Dutschke's former taekwondo studio. Dutschke was earlier seen by the FBI dumping items in the bin he collected from his studio, the affidavit stated.
Traces of ricin were also discovered on four items found at the studio, including liquid removed from a drain and a filter containing items vacuumed from the studio floor, the affidavit said.
When confronted with evidence by federal agents last week about his being observed disposing of items from his studio in the trash, "Dutschke attempted to change the subject, and he ended the interview," according to the affidavit.
Dutschke, 41, was arrested at his Tupelo home on Saturday and was later charged with "developing ... and possessing" ricin and "attempting" to use it "as a weapon," according to a Department of Justice statement.
If convicted, Dutschke faces maximum possible penalties of life imprisonment and a $250,000 fine.
The ricin-laced letters, addressed to Mississippi Senator Roger Wicker, as well as Obama, were retrieved earlier this month at off-site mail facilities before reaching their intended victims. A Mississippi state judge also received a ricin-laced letter.
Discovery of the letters fueled more national anxiety in the days after the bombing at the Boston Marathon.