Ryan Routh, the 58-year-old man accused of plotting to kill Republican presidential candidate Donald Trump at his Florida golf course, pleaded not guilty on Monday to several federal charges.

His lawyer Kristy Militello entered the not guilty plea during a brief arraignment in a West Palm Beach federal courthouse and requested a jury trial.

Wearing a beige prison uniform and shackles on his wrists and ankles, Routh answered "yes, your honor," when the magistrate judge asked him if he was aware of the charges against him.

Routh was arrested on September 15 after a Secret Service agent saw the barrel of a rifle poking out from brush on the perimeter of the West Palm Beach golf course where Trump was playing a round.

The agent opened fire and Routh, who fled in a vehicle, was arrested shortly later.

He has been charged with attempted assassination of a major presidential candidate, assaulting a federal officer and firearms offenses.

A federal judge ruled last week that Routh, identified as a Hawaii resident, should remain in custody.

FBI analysis of Routh's phone showed he had been in Florida since August 18, and his devices were located multiple times between that date and September 15 near Trump's golf course and his Mar-a-Lago residence, according to prosecutors.

Before being spotted by the Secret Service agent, Routh spent nearly 12 hours in the vicinity of the Trump International Golf Club, according to his phone location data.

Court documents said Routh allegedly dropped off a box at an unidentified person's home several months before the attempted assassination containing various letters.

One letter, addressed to "The World," allegedly said: "This was an assassination attempt on Donald Trump but I am so sorry I failed you."

"I tried my best and gave it all the gumption I could muster," it said. "It is up to you now to finish the job; and I will offer $150,000 to whomever can complete the job."

It was the second assassination attempt on Trump this summer. The first took place on July 13 at a campaign rally in Butler, Pennsylvania, when a gunman opened fire on the former president, killing one person and wounding Trump in the ear.

The candidate was otherwise unharmed, and the gunman was killed at the scene.

The Routh case has been assigned at random to federal District Judge Aileen Cannon — a Trump appointee who dismissed criminal proceedings against the former president earlier this year over his retention of top-secret documents at his private residence.