9:30 AM PT -- The U.S. District Attorney's Office for the Southern District of Florida released an affidavit ... in which an FBI agent claims Routh’s public Facebook post from July 10 urged followers to contact him via WhatsApp and points out records apparently placing the phone near the golf course from 1:59 AM to 1:31 PM on September 15 ... suggesting he was scouting the area before the attempted assassination.

The affidavit also notes Routh was seen fleeing in a Nissan SUV, which was later stopped by police. Routh was the sole occupant and acknowledged knowing why he was pulled over, according to authorities.

The Nissan’s license plate was apparently registered to a stolen Ford truck, and the agent says a witness near the golf course identified Routh at the vehicle stop as the person they had seen fleeing the scene.

Ryan Wesley Routh, the man suspected of attempting to assassinate Donald Trump at his West Palm Beach golf course Sunday, made his first appearance in Florida court Monday morning.

Routh was charged with 2 counts -- possession of a firearm by a convicted felon, and possession of a firearm with an obliterated serial number.

Routh will be back in court Monday, Sept. 23, for a formal detention hearing, and is set to be arraigned on Sept. 30.

Further charges were not announced, though they are expected. No shots were fired by the attempted assassin Sunday, according to reports.

Routh was dressed in dark prison scrubs, with his hands and feet shackled, as he made his way into the Paul G. Rogers Federal Building and U.S. Courthouse in West Palm Beach, Florida earlier Monday.

He reportedly sat quietly for about 5 minutes ... showing no visible signs of nervousness, before marshals led him away to await his hearing.

Routh was detained Sunday 50 miles north of the Trump International Golf Club in West Palm Beach ... where law enforcement claims Routh allegedly showed up with an AK-47-style rifle with a scope, 2 backpacks, and a GoPro.

As we've reported, the alleged assassination attempt was thwarted when a Secret Service agent spotted the rifle protruding through a fence about 300-500 yards away from where Trump was golfing, and fired a shot in that direction. The suspect's weapon was not fired. Authorities say Routh fled the scene before officers caught up with him.

Trump's first assassination attempt was back in July, by Thomas Matthew Crooks, who was shot dead at the scene by Secret Service.