WEST PALM BEACH, Fla. — Ryan Routh pleaded not guilty to federal charges in connection with an alleged attempt to assassinate Donald Trump at the former president's golf club in Florida.

Routh, 58, appeared in federal court in West Palm Beach on Monday for his arraignment, which lasted fewer than five minutes.

Routh's lawyers entered a not guilty plea on his behalf.

Magistrate Judge Bruce Reinhart, who notably signed off on the Mar-a-Lago search warrant in the confidential documents case, presided over the arraignment.

Routh had previously been charged with possession of a firearm as a convicted felon and possession of a firearm with an obliterated serial number for the incident that took place at Trump International Golf Club on Sept. 15.

He was indicted last week with three new charges -- attempted assassination of a major presidential candidate, possessing a firearm in furtherance of a crime of violence and assaulting a federal officer -- on top of the two federal firearms charges.

He is being held without bail.

Prosecutors said in court filings there is "probable cause to support additional charges which can and should be considered by the court."

Routh possessed a list that included dates from August to October of venues where Trump had appeared or was expected to be -- and is suspected to have traveled near the golf course and Trump's Mar-a-Lago resort multiple times in the month leading up to his arrest, prosecutors said in a detention filing.

In their memo, prosecutors further revealed Routh allegedly sent a letter "several months prior" to his arrest to a civilian witness that stated, "This was an assassination attempt on Donald Trump but I failed you."

The government argued the sole reason Routh was in West Palm Beach on Sept. 15 was "for one reason and one reason only and that was to kill the former President of the United States."

On the day of the alleged attempted assassination, Trump was playing golf on the course when a Secret Service agent spotted a gun barrel poking out from the tree line near the sixth green, according to investigators.

The agent then fired in the direction of the rifle and saw Routh fleeing the area and entering his nearby vehicle, according to the criminal complaint filed in the case.

In the area of the tree line where the suspect was seen, agents found a digital camera, two bags, including a backpack, and a loaded SKS-style 7.62x39 caliber rifle with a scope, according to the complaint.

Trump was not harmed in the incident and was taken to a safe location by Secret Service agents.

ABC News' Alexander Mallin contributed to this report.

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