The man accused in the apparent assassination attempt of Donald Trump at a golf course in Florida left behind a note detailing his plans to kill the former president, according to prosecutors.
Ryan Routh is accused of keeping in his car a handwritten list of dates and venues where Trump was to appear, the Justice Department said on Monday.
Trump complained that the current holding charges against the man were too light, but prosecutors indicated much more serious charges were coming.
The new allegations about the note were included in a detention memo filed ahead of a hearing Monday at which federal prosecutors argued that Routh should remain locked up as a flight risk.
US Magistrate Ryon McCabe agreed, saying the "weight of the evidence against the defendant is strong" and ordered him to stay behind bars.
The latest details were meant to bolster prosecutors’ claim that the 58-year-old suspect had engaged in a premeditated plan to kill Trump.
The note describing Routh's plans was placed in a box that he dropped off months earlier at the home of an unidentified person who did not open it until after last Sunday's arrest, prosecutors said.
The box also contained ammunition, a metal pipe, building materials, tools, phones and various letters.
The person who received the box and contacted law enforcement was not identified in the Justice Department's detention memo and was described only as a "civilian witness."
In a statement, Trump accused the Justice Department of "mishandling and downplaying" the apparent assassination attempt by bringing charges that were a "slap on the wrist."
The suspect did not fire a shot during the encounter on September 15, standing a few hundred metres away from the Republican presidential candidate who was not in the line of sight.
The suspect fled the scene, leaving behind rucksacks and a GoPro camera after a Secret Service agent spotted the weapon and opened fire. He was later arrested along a Florida highway and has been behind bars since.
Cellphone data showed that the suspect may have been waiting in the area for nearly 12 hours before the gun was spotted, the criminal complaint revealed.
Routh, 58, has not yet entered a plea and has been faced with two federal gun crimes charges.
It is expected more charges could follow as the investigation continues.
Authorities have not yet divulged a motive for the incident, which the FBI has said is being investigated as an apparent attempted assassination of Trump ahead of the November presidential election.
The incident followed the case of another gunman wounding the former president on the ear during a campaign rally in Butler, Pennsylvania, two months earlier.
Gunshots rang through the crowd as Trump sustained a minor injury to his right ear. That gunman was shot and killed by the Secret Service.
The incidents have ratcheted up the already tense partisan nature of the American political system, amplified by extreme rhetoric, ahead of the November election.
