During a search of his Oklahoma City apartment, investigators said they found files on his computer containing a purchase list for pistols, rifles, ammunition and explosive devices.
He was sentenced to eight months in prison after pleading guilty to threatening to kill and inflict bodily harm on the president. A judge later ruled Garnett violated the terms of his supervised release and sentenced him to 24 months to the Federal Bureau of Prisons, recommending that Garnett be put in a facility to let prison authorities evaluate and address his mental health. Garnett was released from federal custody about five months ago.
Authorities have connected Garnett to a Dec. 23 bank robbery attempt in Atlanta, a bank robbery later that same day in Tupelo, Miss., and the shooting death of one Tupelo officer and the wounding of another.
The FBI's special agent in charge in Jackson, Miss., said records showed the suspect's cellphone was in Atlanta, Phoenix and Tupelo at the time of each incident.
The agent, Daniel McMullen, said there were "numerous similarities" between the three bank robberies, including the clothing worn by the suspect and statements he made during the incidents.
Surveillance photos from the attempted bank robbery in Atlanta showed someone wearing a black-and-white patterned jacket tried to rob a Bank of America branch on Dec. 23.
The suspect was wearing a black ski mask and black gloves and carrying a handgun. After failing to get any money from a teller, the person demanded and received an undisclosed amount of money from someone at an ATM.
The suspect was seen leaving in a light gray car with no license plate.
Later that day, someone stole an undisclosed amount of money from the BancorpSouth in Tupelo before fleeing in a gray sedan, the FBI said. Two officers responding to a bank alarm were ambushed by the suspect, McMullen said.
Officer Gale Stauffer was killed, and Officer Joseph Maher was wounded.
On Saturday in Phoenix, someone called 911 to report seeing a man run into the bank while wearing a mask.
There were five people inside the bank during the robbery, Phoenix police spokesman James Holmes said. A customer who ran outside when Garnett turned his attention to the bank manager also called 911.
The first officer to arrive saw the man leave the bank with a bag and a handgun. The suspect then began firing. A detective who also had arrived shot the suspect, who was pronounced dead after being taken to a hospital.
No officers or bank employees were injured.
Holmes said Garnett had served for four years in the U.S. Army. He said Garnett drove a rental car. It was unclear where Garnett had been and what he had been doing while in the state.
Associated Press writer Michael Kunzelman in New Orleans and AP Radio reporter Jackie Quinn in Washington contributed to this report.