Prosecutors in Arkansas say Andrew James McGann left a hiking trail at a state park a bloody mess after stabbing a couple to death … and folks saw him leaving the scene with blood on his face.

In new legal docs, obtained by TMZ, prosecutors go in-depth on the brutal slayings of Clinton Brink and Cristen Brink last month at Devil’s Den State Park in West Fork, Arkansas.

They say a 911 call came in July 26 reporting two dead bodies on a hiking trail with a large amount of blood … the bodies belonged to the Brinks, who were hiking with their young daughters.

ASP says a second witness told them the black sedan was a Kia Stinger … and investigators confirmed as such after poring over surveillance footage from local businesses and homes.

Cops say they found a trail of blood leaving the crime scene that they believed to be the victims’ blood … and an autopsy Monday found they died by homicide from stabbing wounds.

Prosecutors say cops located the Kia Stinger at a barber shop in Springdale on Wednesday afternoon … investigators entered, asked who the car belonged to, and a man getting a haircut — later identified as McGann — indicated he was the owner.

ASP investigators say they noticed McGann had cuts on his hand while speaking with him … and they detained him. They say he gave consent to go into his car to get his ID and when they entered the vehicle, they saw what appeared to be blood inside.

Cops say they seized the car and took McGann to ASP HQ, where “he made statements indicating that he had committed the two homicides.”

McGann is charged with two felony counts of capital murder and prosecutors say he “made a plan to attack and to kill another … then waited with deliberated purpose and killed two hikers at the Devil’s Den State Park.”