Man accused in apparent assassination attempt wrote note indicating he intended to kill Trump: Feds

The man suspected by the FBI of planning to kill former President Donald Trump possessed a list that included dates from August to October of venues where Trump had appeared or was expected to be -- and is suspected to have traveled near the golf course where the incident took place and Trump's Mar-a-Lago resort multiple times in the month leading up to his arrest, prosecutors said in a detention filing on Monday.

In their memo, prosecutors further revealed that suspect Ryan Wesley Routh allegedly sent a letter "several months prior" to his arrest to a civilian witness that stated, "This was an assassination attempt on Donald Trump but I failed you."

Routh, 58, is expected to appear before a judge on Monday. He was ordered to appear in a West Palm Beach federal court for a pre-detention hearing.

Routh has already been charged with possession of a firearm as a convicted felon and possession of a firearm with an obliterated serial number for the Sept. 15 incident that took place at Trump International Golf Club.

RELATED: What we know about the suspect in Trump 'attempted assassination'

According to prosecutors' detention memo, when searching Routh's car following his arrest agents found a total of six cell phones -- one of which contained a google search of how to travel from Palm Beach County to Mexico.

Agents further found "a notebook with dozens of pages filled with names and phone numbers pertaining to Ukraine, discussions about how to join combat on behalf of Ukraine, and notes criticizing the governments of China and Russia."

Cell site records obtained by the FBI confirmed Routh first traveled from Greensboro, North Carolina, to West Palm Beach on Aug. 14, and "on multiple days and times" from Aug. 18 to Sep. 15 his cell phone was located near the golf course as well as Trump's residence at Mar-a-Lago. They did not list the specific dates or the exact number of times Routh is suspected to have traveled near Trump's properties.

Trump was playing golf on the course when a Secret Service agent spotted a gun barrel poking out from the tree line near the sixth green, according to investigators.

RELATED: Timeline of apparent assassination attempt of Donald Trump at golf course

The agent then fired in the direction of the rifle and saw Routh fleeing the area and entering his Nissan vehicle, according to the criminal complaint.

Routh was allegedly 300 to 500 yards away from the former president and did not fire a shot, according to investigators. Trump was not in Routh's line of sight, according to the Secret Service.

Witnesses reported the license plate number to authorities, and the suspect was stopped and detained.

In the area of the tree line where the suspect was seen, agents found a digital camera, two bags, including a backpack, and a loaded SKS-style 7.62x39 caliber rifle with a scope, according to the complaint. The serial number on the rifle "was obliterated and unreadable to the naked eye," the complaint states.

Routh did not enter a plea for his initial charges and his arraignment is scheduled for Sept. 30.

The investigation is ongoing and the FBI has been going through Routh's social media and criminal history and speaking with family members to get more clues.

Investigators further revealed that three days after they arrested Routh, a civilian witness contacted them with information that Routh had dropped off a box at their residence several months ago that contained "ammunition, a metal pipe, miscellaneous building materials, tools, four phones, and various letters."

RELATED: Apparent assassination attempt | Suspect never had Trump in sight, did not get off shot: Officials

"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 said. "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."

Routh suggested Iran should feel "free to assassinate Trump" and himself in a self-published book from February 2023.

In the book, which ABC News has unearthed following Sunday's incident, Routh directed an apology toward Iran, apparently for his previous support for Trump, who withdrew the U.S. in 2018 from the Joint Comprehensive Plan of Action (JCPOA) nuclear deal with Iran.

Agents further said they had reviewed a book authored by Routh, which has previously been publicly reported on, titled, "Ukraine's Unwinnable War: The Fatal Flaw of Democracy, World Abandonment and the Global Citizen-Taiwan, Afghanistan, North Korea, WWIII and the End of Humanity."

In it, Routh wrote he "must take part of the blame for the [person] that we elected for our next president that ended up being brainless, but I am man enough to say that I misjudged and made a terrible mistake and Iran I apologize. You are free to assassinate Trump as well as me for that error in judgment and the dismantling of the deal. No one here in the US seems to have the balls to put natural selection to work or even unnatural selection."

RELATED: Joe Biden says Secret Service 'needs more help' after apparent Trump assassination attempt

The suspect was also a booster of a number of causes, including the war in Ukraine, an ABC News analysis of his apparent social media profiles shows.

As authorities try to unravel the motive and details of the case, sources said investigators were looking at whether Routh was frustrated with Trump's position on Ukraine.