Yesterday, Keel, 44, of the Far Northeast, pleaded guilty in federal district court to threatening the staffer.
Authorities have not identified the victim, referred to in the government's plea memo by the initials M.S., who was a summer intern and is now in college.
Assistant U.S. Attorney Robert Reed and federal defender Catherine Henry negotiated a plea deal in which Keel will serve three years' probation and participate in a mental-health program.
The plea deal must still be approved by U.S. Magistrate Timothy Rice, who set sentencing for Keel for May 23. He is free on bail.
The plea memo said that after speaking with M.S., Keel called the congresswoman's office back and spoke with a female staffer, who asked Keel where he was.
Keel said he was at a local hospital and the female staffer called authorities to report the threat. By the time federal agents arrived to interview him, however, Keel had been discharged.
Agents then spoke with hospital staff and learned that while Keel was there, he was screaming and told staff he would call his congressman and lawyer to complain about lost dentures.
Later that evening, three FBI agents went to Keel's home on Academy Road near Brandon.
During an interview, the plea memo said, Keel told the agents he had called Schwartz's office, and spoke with a staffer to complain about the hospital and his lost dentures. Keel stated that someone needed to get shot, referring to the hospital staff, the plea memo said. He was arrested two days later.