The three young adults charged with murdering Richel Nova earlier this year discussed a plan to rob someone hours before they lured him to a vacant home and stabbed him to death, a Suffolk County prosecutor said at their arraignment in Suffolk Superior Court today.

Assistant District Attorney Jennifer Hickman of Suffolk DA Daniel F. Conley’s Homicide Unit today gave the most detailed account yet of the events leading to Nova’s death just after midnight on Sept. 2, asking that ALEXANDER GALLETT (D.O.B. 4/28/92) of Hyde Park; his girlfriend, YAMILEY MATHURIN (D.O.B. 2/25/93) of Mattapan; and MICHEL ST. JEAN (D.O.B. 8/3/90) of Hyde Park remain behind bars pending trial.

“What we’ve learned is chilling,” Conley said. “This was a crime that should shock the conscience of any decent person.”

Suffolk Superior Court Clerk Magistrate Gary D. Wilson granted that request and ordered all three to return to court on Dec. 16. The three were indicted yesterday on charges of first-degree murder for stabbing Nova to death; armed robbery for stealing his cash, his car, and the food he had brought them; and breaking and entering with the intent to commit a felony for gaining unlawful access to a second floor apartment at 742 Hyde Park Ave., where Nova was slain.

On the afternoon of Sept. 1, Hickman said, a friend of the defendants heard all three of them discussing robbing someone. That witness also heard St. Jean say that “he was looking for a victim,” the prosecutor said.

“Yamiley Mathurin was the face of this crime,” Hickman said of the lone female defendant. “Its success depended on her performance and interaction with people … It was the role of Yamiley Mathurin to get Richel Nova up to the second floor.”

Mathurin allegedly borrowed from a neighbor the cell phone from which Nova’s killers made the call for a delivery. She allegedly requested the delivery and later called back because it was late. Hickman said she was described as “innocent,” “sweet,” and “not agitated” by witnesses who observed her demeanor and listened to her voice.

Hickman said Mathurin specifically asked if the driver would be able to make change for a large bill and was told that drivers carry only $20 in change. She ordered two pizzas, chicken wings, and soda, totaling about $32. A Domino’s employee stated that a driver would not be able to break a $100 bill but could give change on that order from a $50 bill.

Nova arrived at the address shortly before 11:40 p.m. and called St. Jean’s cell phone – the number Mathurin had given as a call-back number. The calls were not answered. At about that time, a nearby resident saw Mathurin talking to a deliveryman near the building’s back stairs, then leading him up those stairs. A short time later, the neighbor “heard a commotion coming from the vacant house and heard someone call out in Spanish,” Hickman said.

The neighbor and another man soon went to 742 Hyde Park Ave. to check on the deliveryman’s safety. They found Nova on the floor.

“His eyes were open,” Hickman said. “He did not appear to be alive. He was lying on the floor with a massive amount of blood coming from his neck. There were chicken wings on the floor.”

The witnesses called 911 and emergency medical technicians arrived minutes later. They pronounced Nova dead at 12:04 a.m. He had been stabbed 16 times in the chest, back, arms, neck, and throat. His pockets had been turned out.

Nova’s vehicle was located the next day in the parking lot of a River Street church.

“It appeared that someone had poured bleach on the steering wheel and other parts of the car,” Hickman said, noting that a bloody Domino’s pizza box with three slices of pizza was recovered from underneath an adjacent car.

In interviews with Boston Police homicide detectives on Sept. 3, each of the three allegedly gave detailed statements in which they acknowledged their presence at 742 Hyde Park Ave. when Nova was killed. Based on those statements, cell phone records, witness statements, and other evidence gathered during a round-the clock-investigation, Suffolk prosecutors approved warrants for their arrests.

Catherine Rodriguez is the DA’s assigned victim-witness advocate. Gallett, Mathurin, and St. Jean are represented by attorneys Peter Krupp, Steven Sack, and Robert Jubinville, respectively.