The man accused of attempting to assassinate former President Donald Trump on his golf course in West Palm Beach, Florida, left a handwritten manifesto confessing to the plot and offering money for someone else to finish the job if he failed.

“This was an assassination attempt on Donald Trump but I failed you,” the manifesto said, according to a court filing by prosecutors. “I will offer $150,000 to whomever can complete the job.”

Ryan Wesley Routh, 58, charged with two felony firearms violations, was denied bail in a Miami federal court Monday.

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Prosecutors said they would seek an additional federal charge of attempted assassination, which carries a sentence of up to life in prison. Routh, a convicted felon, is scheduled to be arraigned on Sept. 30.

In court documents released before the bail hearing, prosecutors revealed evidence that strengthens the case Routh was seeking to assassinate Mr. Trump on Sept. 15 when he hid in the bushes along the sixth hole of Trump International Golf course with an SKS-style rifle.

In addition to the manifesto, which was left in a box at the home of an acquaintance, prosecutors found Routh’s fingerprint on the rifle, which was loaded with 11 rounds and had a round in the chamber.

Data recovered on his phone showed he searched driving routes to Mexico.

Other equipment in Routh’s Nissan Xterra included license plates, six cellphones, 12 pairs of gloves and a passport. His backpack was stocked with plates “capable of stopping small arms fire.”

In addition to the manifesto, investigators discovered a handwritten list of dates in August, September and October that listed places where Mr. Trump had appeared or was expected to appear.

Former federal prosecutor Neama Rahmani said Routh’s note outlining his plan to try to kill Mr. Trump will make it easy for a grand jury to indict him on charges of attempted assassination.

“The note establishes attempt. It doesn’t get better than the defendant’s own words,” Mr. Rahmani said.

One page of the note was shown in court documents. It was addressed “Dear World” and appeared to lament a failed assassination attempt on the former president.

“Everyone across the globe from the youngest to the oldest know that Trump is unfit to be anything, much less a U.S. president,” the note said.

Mr. Trump, he wrote, “ended relations with Iran like a child and now the Middle East has unraveled.”

Routh never fired a shot at the former president but got within several hundred yards with the semiautomatic rifle.

Routh was spotted by a Secret Service agent in the bushes directly across from the sixth hole at the golf course, where the former president was playing.

The agent was sweeping the area in advance of Mr. Trump, who was close behind at the fifth hole, according to the court document. The agent saw the barrel of a rifle protruding through the bushes and a man’s face partly obscured by the foliage. The gun barrel shifted toward the agent, and the agent fired his service weapon at the man, prosecutors said in court documents.

Criminal defense lawyer David Tarras of the Fort Lauderdale-based Rossen Law Firm said prosecutors could use Routh’s note as direct evidence of his intent when he was discovered on the golf course.

“Learning that the former President was approaching the hole where the sniper’s nest was located is also likely to be used as strong circumstantial evidence for the government to meet their burden that this was more than mere planning, and included a substantial step towards completion of the assassination,” Mr. Tarras said. “The closer the crime was to being carried out, the better case the government has for proving an attempt-related offense, like attempted assassination.”

After the Secret Service fired at Routh, a witness spotted him fleeing the bushes, getting into his car and speeding away from the scene. He was arrested later, and his rifle with scope was found in the bushes along the sixth hole. Mr. Routh’s fingerprint was lifted from the gun, court documents indicate.

The court documents revealed that Routh dropped off a box at the home of an unidentified witness several months before the assassination attempt. The witness opened the box after Routh’s arrest and found ammunition, a metal pipe, building material tools, four phones and various letters, including the manifesto about attempting to kill Mr. Trump.

Cellphone tower data shows Routh visited the area near the golf course “on multiple days and times” one month before the Sept. 15 incident.

State prosecutors, under the direction of Florida Gov. Ron DeSantis, a Republican, are conducting an independent investigation that could lead to a charge of attempted murder, which carries a life sentence in Florida.

It was the second attempted assassination of Mr. Trump in two months.

The former president was grazed in the ear by an attempted assassin’s bullet on July 13 at a rally in Butler County, Pennsylvania. The shooter, positioned on a nearby roof, killed a rally attendee and injured two others before he was killed by a Secret Service sniper.

Mr. Trump on Monday questioned the impartiality of the Justice Department in carrying out the prosecution of Routh. Special counsel Jack Smith charged Mr. Trump in two criminal cases involving interference in the 2020 election and storing classified documents at his home in Palm Beach. The former president was convicted on felony business fraud charges in a hush-money case in New York and is facing felony charges in Fulton County, Georgia, related to election interference in 2020. Mr. Trump said all the cases are political lawsuits carried out or influenced by the Biden administration.

In a lengthy statement, Mr. Trump also blasted FBI Director Christopher A. Wray, who, shortly after the Butler County assassination attempt, told Congress that Mr. Trump may not have been hit with a bullet and may have been struck by glass or shrapnel. Photos show a bullet whizzing by Mr. Trump and no evidence that the teleprompter glass was broken or nicked.

“The DOJ and FBI have a Conflict of Interest since they have been obsessed with ‘Getting Trump’ for so long,” Mr. Trump said. “It’s very difficult to trust the Biden/Harris DOJ/FBI to investigate the assassination attempts, due to Election Interference and the FAKE CASES brought against me, including their control over local D.A.s and A.G.s.”

Mr. Trump said federal prosecutors should step aside and “let Florida handle the case.”