Anton Caluori was arrested on Tuesday in north-western Washington state for the alleged threats.
The US attorney's office in Seattle confirmed on Wednesday that he had been discharged by the US Navy within the past 10 years.
Prosecutors said the threat was sent to a generic FBI email address.
Mr Caluori faces up to five years on the threat charge and up to 20 years for allegedly assaulting a federal officer. He will appear in court again on Monday for a detention hearing.
Officials detained Mr Caluori, 31, when he answered the door at his home in the city of Federal Way, allegedly armed with a shotgun.
According to court charging papers, police officer Andy Hensing grabbed the barrel of gun before any shots were fired, a Secret Service agent wrote in a complaint.
"Officer Hensing grabbed the gun barrel as Caluori lifted it in our direction, and I grabbed Caluori's person," Special Agent Bryan Molnar said.
The gun was loaded to capacity, including a round in the chamber, and the safety was off, he said.
According to the complaint, officials found a AK-47 style rifle in the kitchen, with eight magazines nearby, as well as a AR-15 rifle loaded with a 100-round drum magazine.
The two officials went to Mr Caluori's apartment after the FBI had notified the Secret Service that it received an "alarming" email regarding the president.
"I will kill the president!!!!!," the email reportedly read, "'I' want you to come and get me ... you can't afford to call my bluff."
Mr Caluori appeared in a US district court in Seattle on Wednesday with several cuts and scratches on his arms.
His lawyer, Kyana Stephens, said she intended to request a psychological examination.
A man accused of threatening US President Barack Obama by email and showing a shotgun to officers at his door has appeared in court.