The man accused of staking out Donald Trump’s Florida golf course with a rifle has been indicted on a charge of attempted assassination of a political candidate as prosecutors allege he intended to kill the former US president.

Ryan Routh, 58, was already facing two gun-related charges after authorities said he pointed a rifle through a fence at Trump’s golf club in West Palm Beach, Florida on September 15 while the Republican presidential candidate was golfing there.

He has been ordered to remain in jail to await trial.

A federal prosecutor said on Monday (local time) that the US Justice Department would ask a grand jury to approve the more serious attempted assassination charge, which carries a maximum of life in prison.

On Tuesday, US Attorney General Merrick Garland called the attempted assassination a “heinous act” and said the Justice Department would “spare no resource” on the case.

Routh has not yet entered a plea. His lawyers unsuccessfully sought to have him released on bond.

Prosecutors have in recent days revealed evidence they said pointed toward a plan to kill Trump. They allege that months before the incident, Routh dropped off a letter to an unidentified person alluding to “an assassination attempt on Donald Trump”.

They said Routh spent a month in South Florida and mobile phone data showed him near the golf course and Trump’s Florida residence at Mar-a-Lago. He was found with a handwritten list of dates and venues where Trump spoke or was expected to appear, according to court filings.

A US Secret Service agent searching the golf course ahead of Trump opened fire after discovering the gun poking through the fence, causing Routh to flee, prosecutors said. He was arrested on a Florida highway within an hour.

Routh was initially charged with possessing a firearm as a convicted felon and possessing a firearm with an obliterated serial number.

It was the second attempt on Trump’s life in just weeks, following an earlier shooting at a rally in Pennsylvania.