Gunman left note in case Trump assassination attempt failed

“Dear World, This was an assassination attempt on Donald Trump but I am so sorry I failed you," the handwritten note from Ryan Wesley Routh said. “I tried my best and gave it all the gumption I could muster. It’s up to you to finish the job; and I will offer $150,000 to whomever can complete the job."

Federal prosecutors included a photo of the note in a new court filing as they urge a judge to keep Routh detained while his court case proceeds.

A witness found the note inside of a box that Routh had dropped off months before the Sept. 15 golf-course encounter. The witness opened the box, which also contained ammunition, tools and a metal pipe, after learning that Routh had been accused of trying to kill Trump.

The note was one of several new revelations about Routh’s plans in the months and weeks before a Secret Service agent allegedly saw him pointing a semiautomatic rifle through a golf-course fence while Trump was playing a few holes away.

Routh had compiled a handwritten list of dates and places where he expected the Republican nominee to be present, and had arrived in Florida more than a month before the encounter, prosecutors said. Cellphone records showed he had on multiple days traveled near the Trump International Golf Club in West Palm Beach—about 5 miles from Trump’s Mar-a-Lago residence—between Aug. 18 and the incident on Sept. 15.

Routh, 58, is expected in federal court later Monday morning, where prosecutors will point to those details as evidence he should remain jailed.

Officials said Routh hid undetected near the golf course for nearly 12 hours that day, before the Secret Service agent spotted him and opened fire. Authorities don’t think Routh fired a shot. He had been roughly 400 yards away from the Republican presidential nominee, but didn’t have a direct line of sight to him, officials said. He sped off in a black Nissan but was captured soon after as he fled north on Interstate 95.

Authorities found a loaded semiautomatic SKS-style rifle with a scope, two backpacks and a bag of food in the area where the suspect had been hiding near the course. On Monday, prosecutors revealed they had also found two additional license plates and six cellphones, one of which contained a google search for how to travel from West Palm Beach to Mexico, an indication that he had hoped to flee.

The rifle had a serial number that had been scratched off, a wrinkle that has made it difficult for law-enforcement officials to quickly determine how the felon with a long criminal record was able to obtain it.

Investigators also found a notebook with dozens of pages filled with names and phone numbers pertaining to Ukraine and notes criticizing the governments of China and Russia, prosecutors said. Routh had been a pro-Ukraine activist for years, traveling there shortly after the Russian invasion in 2022 in the hopes of joining the fight.

The close call came as the American public is increasingly on edge amid growing threats of politically motivated violence ahead of November’s election. Evidence that the gunman went unnoticed near the premises for hours also raised fresh doubts about the Secret Service’s ability to protect both presidential candidates during such a volatile campaign season.

Write to Sadie Gurman at sadie.gurman@wsj.com and Arian Campo-Flores at arian.campo-flores@dowjones.com