The letter was dropped off at the home of a witness several months before the 15 September incident in Florida.
A suspected gunman arrested near Donald Trump’s golf-course wrote a note months earlier saying he intended to kill the former president, a court filing shows.
“This was an assassination attempt on Donald Trump,” the note says.
In court documents filed on Monday, prosecutors said the letter was dropped off at the home of a witness several months before the 15 September incident in Florida.
Ryan Routh, 58, is expected to appear at a federal court later on Monday and could face further charges.
He is so far facing two federal gun crime charges.
The pre-written note appears to pre-empt a failed assassination attempt. “I tried my best and gave it all the gumption I could muster,” it reads.
The court documents filed by prosecutors in support of Mr Routh’s continued pre-trial detention noted that a box containing the letter – as well as ammunition, building materials, tools and four phones were dropped off at the home of the unnamed witness before the incident.
Mr Routh has been held in jail since his arrest on 15 September. A Secret Service agent spotted his face in foliage while securing the sixth hole of Trump’s golf course in West Palm Beach before noticing a rifle, prosecutors say.
The agent then jumped out of his golf cart, drew his gun and fired after seeing Mr Routh allegedly move his gun, the court filing says.
The court documents also indicate he had 11 rounds of ammunition, one of which was chambered in the rifle.
Phone records show that Mr Routh had been near Trump’s Mar-a-Lago resort for nearly a month between 18 August and 15 September.
He is originally from North Carolina and spent much of his life there, although he most recently lived in Hawaii.
In 2022, he went to Ukraine in a failed bid to recruit foreign soldiers for Ukraine’s military following the country’s invasion by Russian forces.
The American reportedly contacted the legion on a regular basis with ideas described by one Ukrainian soldier as “nonsensical” and “delusional”.
The suspect had also admitted to being turned down himself, claiming it was down to his age and lack of fighting experience.