West Palm Beach (United States) (AFP) - The gunman suspected of planning to kill Donald Trump on his golf course wrote a chilling letter months ago about a failed “assassination attempt” on the former US president, according to court papers filed Monday as he was remanded in custody.

The correspondence emerged as Ryan Routh appeared before a judge in south Florida where he was charged on suspicion of two firearms offenses, with prosecutors indicating that they planned to seek a further count of attempted assassination.

Routh, 58, was arrested allegedly fleeing in a car from Trump International Golf Course in West Palm Beach, Florida on September 15.

A Secret Service agent had fired on him after spotting a rifle poking out of the tree line several holes ahead of Trump, the government alleges in a preliminary filing to the court.

The document says Routh had allegedly 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 am so sorry I failed you,” said the letter, the front page of which was pictured in an image included with the filing.

“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 $150,000 to whomever can complete the job.”

It was not clear if he was referring to a separate, earlier attempt on Trump’s life or if he was predicting that his golf course assassination bid would end in failure.

Routh was remanded in custody at his hearing in West Palm Beach.

He is accused of keeping a gun with a scratched-out serial number and possessing a firearm illegally as a felon, although prosecutors said they would seek an “attempted assassination” charge when they put the case before a grand jury.

This would bring a maximum penalty on conviction of life imprisonment.

- ‘Slap on the wrist’ -

Trump accused the Department of Justice and FBI of “mishandling and downplaying” the golf course incident – alleged to be the second attempt on Trump’s life after a shooting at a Pennsylvania rally in July.

The gunman in the earlier case was killed at the scene and it is not thought to be connected with the new alleged plot.

“The charges brought against the maniac assassin are a slap on the wrist,” Trump said in a written statement which may have been prepared before it emerged that extra felony counts were a possibility.

In a statement littered with falsehoods and baseless conspiracy theories about his various criminal cases, Trump called for the Routh case to be handed to state authorities in Florida, which is run by hard-right Republican governor Ron DeSantis.

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 traveled to Florida a month before his arrest and began watching 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.

“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’s defense team acknowledged that he had been arrested on less serious allegations in the past but described him as a law-abiding citizen with no record of violence.