The man accused of attempting to assassinate former President Donald Trump at his Florida golf course earlier this month pleaded not guilty Monday to federal charges.Ryan Wesley Routh, 58, was arrested after allegedly pointing a high-powered rifle through a fence near where Trump was golfing at his club in West Palm Beach, Florida, on Sept. 15. RELATED STORY | Secret Service: Suspect in apparent Trump assassination attempt didn't fire weaponRouth was initially charged with gun-related crimes, including illegal possession of a firearm and obliterating a firearm's serial number. The Justice Department later added the charge of attempted assassination of a presidential candidate.Evidence shared in court included a letter allegedly written by Routh that offered a reward of $150,000 to anyone who could kill Trump.According to prosecutors, the note said "This was an assassination attempt on Donald Trump but I failed you. 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."A civilian witness notified authorities on Sept. 18 of a box dropped off by the Routh at his residence several months prior. He reportedly felt compelled to open it after the events that unfolded at Trumps golf course.RELATED STORY | Son of man accused in Trump assassination attempt arrested on child porn chargesIn addition to that letter, the box contained several other letters, ammunition, a metal pipe, various building materials, tools, and four phones, according to the Department of Justice.Investigators are still determining a timeline of the events and how Routh was able to get within several hundred yards of the former president. If convicted on the most serious assassination attempt charge, he could face up to life in prison.