Metro

Man wounded during fatal drug rip-off in East Boston charged with armed robbery

A man who was shot and wounded while allegedly participating in a drug rip-off that cost an East Boston man his life is now facing charges for the armed robbery that ended in the murder of Duncan Ketter.

Demetrius Cast is scheduled to be arraigned on Wednesday in his hospital bed at Massachusetts General Hospital, where is being treated for gunshot wounds suffered in East Boston on Sunday. Cast, 20 of Everett, is the third person under arrest for the murder of Ketter, 21.

Ketter was fatally shot around 4:45 p.m. Sunday while he and a female acquaintance were trying to complete the sale of 3 ounces of marijuana for $550, according to Suffolk District Attorney Daniel F. Conley’s office.

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Prosecutors and Boston police allege that a juvenile negotiated the deal with the female acquaintance and that all three got into an SUV where the marijuana was supposed to be exchanged for the cash.

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Instead, according to law enforcement, 18-year-old Thorus O’Brien “popped” up from the rear seat of the SUV where he had been hiding, pointed a gun at Ketter and demanded that he hand over the drugs to him.

When Ketter “verbally argued,’’ O’Brien allegedly shot him in the back and then pushed his body out of the SUV near a snowbank near the intersection of Marginal and Orleans streets. The female acquaintance and the juvenile were also ordered out of the car, prosecutors allege.

Cast, whose role in the armed robbery was not spelled out by law enforcement on Wednesday, was shot in the arm “during this incident,’’ according to a Boston police report filed in East Boston Municipal Court.

He has been hospitalized since Sunday, according to the report.

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The 16-year-old juvenile who negotiated the drug sale was arraigned Monday in the Suffolk County Juvenile Court on a charge of delinquency, to wit: armed robbery and armed assault with intent to rob in connection with the incident. His bail was set at $100,000.

O’Brien is being held without bail after pleading not guilty to murder and other charges at his arraignment in the East Boston courthouse Monday.

John R. Ellement can be reached at ellement@globe.com. Follow him on Twitter @JREbosglobe.