An 18-year-old fatally shot a St. Paul man in the back last week shortly after the man robbed him during a marijuana deal, according to criminal charges.

Darrion Isaiah Gresham, also known as Darrion Gresham Coats, was arrested last weekend after others allegedly involved — as well as video surveillance footage — revealed him to be the shooter, according to a criminal complaint filed in Ramsey County District Court.

He is charged with two counts of second-degree murder in the death of Larnell Brown, a 42-year-old father.

A man who had been with Brown last Tuesday told police that the two met with Gresham to buy marijuana from him, the complaint said.

They climbed into Gresham’s BMW to conduct the exchange, according to the complaint, and drove to a liquor store near University Avenue and Arundel Street.

That’s when Brown and his friend pulled out guns and robbed Gresham, according to criminal charges.

Brown took marijuana from a bag that belonged to Gresham, then grabbed the keys to Gresham’s BMW, and got out of the vehicle along with his accomplice.

As the two men walked away down an alley, gunshots rang out. Brown’s friend started running, and Brown was hit, according to the complaint.

The friend fired back toward Gresham, and then ditched his gun, according to the complaint.

Surveillance footage shows Brown make contact with someone else nearby and hand over his gun. Then he went across the street and collapsed, according to the charges.

Metro Transit police who were on patrol in the area heard the gunshots. One of them saw a man wearing a black jacket standing in the sidewalk of Arundel Street holding a handgun in his extended left hand as he fired down the street, according to the criminal complaint.

Shortly thereafter police discovered Brown, keeled over and holding his hands to his chest.

Brown was taken to Regions Hospital, where he was pronounced dead at 4:33 p.m.

Five 9mm casings were found near the vehicle, and police towed the BMW.

Suspecting Gresham’s involvement, one of the officers called a number associated with him and heard a phone ring inside the vehicle, according to the criminal complaint.

Police also interviewed both men seen with Brown that night.

The man who said he was an accomplice to the robbery identified Gresham from a lineup as the shooter with 50 percent certainty the first time around, and 70 percent the second time, according to the charges.

Police later recovered “high-quality” surveillance footage of the incident from a nearby store, and officers familiar with Gresham recognized him as the driver of the BMW that day, the complaint said.

Officers also were able to find Brown’s and his accomplice’s discarded handguns.

The Ramsey County medical examiner determined Brown died from a gunshot wound to the back.

Gresham’s next court appearance on the charges is scheduled for Feb. 10.