US prosecutors have charged a man arrested near Donald Trump’s Florida golf course with the attempted assassination of a presidential candidate.

It comes a day after a court filing showed Ryan Wesley Routh had written a note months ago saying he intended to kill Trump.

Routh, 58, already faces up to 20 years behind bars on two gun-related charges.

But he faces a maximum sentence of life in prison if convicted on the new, more serious charge.

“The attempted assassination on the former president is a heinous act,” Attorney General Merrick Garland told reporters on Tuesday, vowing he would “spare no resource to ensure accountability”.

Routh has been ordered to remain in jail since being taken into custody on 15 September in Florida.

Earlier that day, a Secret Service agent had spotted his rifle poking through a fence at Trump International Golf Club in West Palm Beach.

The agent opened fire as the three-time Republican presidential nominee was playing a round of golf.

Routh fled without firing a shot, leaving his weapon and other items at the scene as agents escorted Trump to safety.

He was arrested shortly afterwards, after a witness spotted him on the highway.

In addition to the attempted assassination charge, an eight-page indictment against Routh unsealed on Tuesday also charges him with:

• possessing a firearm in furtherance of a crime of violence;
• possessing a firearm as a felon;
• possessing a firearm with an obliterated serial number;
• assaulting a federal officer

This is a breaking news story. Stay tuned for updates.