Miami, Florida - The suspect in an apparent effort to kill Donald Trump has been indicted on an attempted assassination charge, US authorities said on Tuesday.

According to allegations in court documents, on September 15 a US Secret Service agent "saw what appeared to be a rifle poking out of the tree line" while Trump was playing at his golf course in Florida.

After the agent fired in the direction of the suspected gun, a witness saw a man later identified as 58-year-old Ryan Wesley Routh fleeing the area. Routh escaped in a vehicle and was later apprehended by police.

The 58-year-old was initially charged with possession of a firearm by a convicted felon and possession and receipt of a firearm with an obliterated serial number.

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Routh had now been charged with attempted assassination of a presidential candidate, "which strikes at the very heart of our democratic system," FBI director Christopher Wray said.

Attorney General Merrick Garland said violence targeting public officials endangered everything the US stood for.

"The Justice Department will not tolerate violence that strikes at the heart of our democracy, and we will find and hold accountable those who perpetrate it. This must stop."

According to court documents, Routh left a note outlining his intentions. The handwritten letter address "Dear World," stated, among other things, "This was an assassination attempt on Donald Trump but I am so sorry I failed you."

If convicted, Routh faces a maximum penalty of life in prison.

At a campaign event in Pennsylvania in July, Trump was injured by a bullet to his right ear and the shooter was shot dead by security forces shortly afterward. One man died and two others were injured at the rally.