BOSTON, Nov. 1, 2012—A Dorchester man was arraigned today for the murder of 23-year-old Alfonso Rivas during an attempt to rob the victim of marijuana, District Attorney Daniel F. Conley said.

OASIS PRITCHETT (D.O.B. 10/5/90) was held without bail at his arraignment before Dorchester District Court Judge Robert Tochka on charges of murder, armed robbery, and unlawful possession of a firearm in Rivas’ Oct. 17 shooting death.

Assistant District Attorney Ursula Knight told the court that Pritchett and another man planned to rob Rivas of a quantity of marijuana during a transaction at the Lyndurst Street building in which Rivas lived. Rivas was shot in the head, suffering fatal injuries, during that robbery and his iPhone was also taken.

Cell phone records show that Rivas took a phone call just before he was slain, Knight told the court, and surveillance images from the building show Pritchett and the second man speaking on a cell phone as they entered 14 Lyndhurst St. The surveillance images also show Pritchett and the second man fleeing four minutes later.

Knight said surveillance images and Charlie Card records track Pritchett from Lyndhurst Street to an MBTA station and onto a bus that took him past the murder scene. In a post-Miranda statement, Pritchett allegedly admitted that he was present at the scene.

Rivas, whose girlfriend and children were in an adjoining apartment during the shooting, was transported to Boston Medical Center where he was later pronounced dead.

Pritchett was represented by Jonathan Shapiro. He is due back in court on Nov. 29.

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All defendants are presumed innocent until and unless proven guilty beyond a reasonable doubt.