Federal prosecutors on Monday said in a new court filing that Ryan Routh, the accused would-be assassin of Donald Trump, stalked the ex-president for a month before being arrested last week outside of his Florida golf course.

The filing says that on Aug. 14, "Routh traveled from the Greensboro, North Carolina, area, to West Palm Beach, Florida."

"On multiple days and times from August 18, 2024, to September 15, 2024, Routh's cell phone accessed cell towers located near Trump International and the former President's residence at Mar-a-Lago," according to the filing in U.S. District Court for the Southern District of Florida.

Routh is charged with possession of a firearm by a convicted felon and possession of a firearm with an obliterated serial number.

The filing, made in advance of Routh's scheduled court appearance on Monday, also details the contents of a handwritten letter that was in a box that a witness opened on Wednesday. The witness told investigators that Routh had dropped off the box several months before Sept. 15. The box "contained ammunition, a metal pipe, miscellaneous building materials, tools, four phones, and various letters," the filing said.

"One handwritten letter, addressed to "The World," stated, among other things, 'This was an assassination attempt on Donald Trump but I failed you,' " the filing revealed.

" ' I tried my best and gave it all the gumption I could muster,' " the letter said, according to the court filing.

Prosecutors also revealed that investigators had found, in a backpack and shopping bag linked to Routh, plates that "were capable of stopping small arms fire."

And "during a search of the Nissan Xterra" that Routh had used, "FBI agents found two additional license plates," the court filing said.