WASHINGTON, United States (AFP) — The gunman suspected of planning to kill Donald Trump wrote a chilling letter months ago about an “assassination attempt” on the former president, United States (US)  prosecutors said on Monday.

The correspondence emerged as Ryan Routh, 58, was due in court in southern Florida charged with two firearms offenses after he was arrested allegedly fleeing Trump International Golf Course in West Palm Beach on September 15.

Prosecutors say a Secret Service agent fired at Routh after spotting a rifle poking out of the tree line several holes ahead of Trump, and that Routh was spotted by a witness who saw him escaping in a car.

Federal prosecutors said in Monday’s filing Routh had dropped off a box at an unidentified person’s home “several months” beforehand, containing ammunition, a metal pipe, building materials, phones “and various letters”.

After learning of the apparent assassination bid, the witness apparently opened the box and discovered a letter addressed to “The World”.

“This was an assassination attempt on Donald Trump but I failed you,” it said.

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

It was not clear if he was referring to a separate, earlier attempt on the life of Trump or the one at the golf course.

Detectives believe Routh may have been lying in wait for Trump for nearly 12 hours at the Florida course. He was arrested after police spotted his car driving on a nearby highway.

The government says Routh travelled to Florida a month before his arrest and began casing the golf course and drove by Trump’s oceanfront home.

In online posts Routh said he voted for Trump in 2016 but that the president had become a disappointment.

“Everyone across the globe from the youngest to the oldest know that Trump is unfit to be anything, much less a US president,” said Routh’s letter, the front page of which appeared in a photograph included with the court papers.

“US presidents must at bare minimum embody the moral fabric that is America, and be kind, caring and selfless and always stand for humanity. Trump fails to understand any of (that).”

Routh is accused of possessing a gun with a scratched-out serial number and with possessing a gun illegally as a felon, although more serious charges may come after prosecutors put the case before a federal grand jury.