Washington DC - The Department of Justice has released a letter allegedly written by the man facing charges for attempting to assassinate Donald Trump.

On Monday, DOJ prosecutors filed a "Written Factualy Proffer" in support of their request that Ryan Routh, the man who was caught allegedly attempting to assassinate the former president on September 15, be placed in detention ahead of his trial.

The document revealed that an unnamed witness claimed Routh "dropped off a box at his residence several months prior" to the incident.

After learning of Routh's arrest, the witness opened the box, discovering "ammunition, a metal pipe, miscellaneous building materials, tools, four phones, and various letters."

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The DOJ included an image of one of the letters, addressed to "the world," in which Routh allegedly wrote, "This was an assassination attempt on Donald Trump, but I am so sorry I failed you."

"I tried my best and gave it all I've got – but I couldn't muster it," the letter continued. "It's up to you now to finish the job, and I will offer $150,000 to whomever can complete the job."

The letter goes on to argue that Trump is "unfit to be anything, much less a US president" and claims that Trump's choice to end relations with Iran caused the Middle East to become "unraveled."

On September 15, Secret Service agents opened fire after they spotted what appeared to be a gunman, now believed to be Routh, with a rifle, hiding in the bushes at one of the former president's golf courses in Florida.

Routh fled the scene and was later tracked down and arrested by authorities.

The DOJ's filing claimed that cell phone data revealed Routh had been waiting at the golf course for nearly 12 hours, he had been in the area and near other places Trump frequents multiple times within the month, and he had Google searched best travel routes from Florida to Mexico before the incident.

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He also allegedly had a handwritten list of various dates and places Trump was scheduled to appear, leading up to the elections in November.

Routh, who is facing charges for possession of a firearm as a convicted felon and possession of a firearm with an obliterated serial number, will appear in court on Monday for the detention hearing.