The young man who allegedly fired a bullet into 18-year-old Paula Castillo’s neck, killing her and the fetus she was carrying, was ordered held without bail today at his arraignment on murder and other charges, Suffolk County District Attorney Daniel F. Conley said.

RUBEN SANCHEZ (D.O.B. 9/6/90) of Hyde Park was arrested late Friday for Castillo’s murder, unlawful possession of a firearm, and discharging a firearm within 500’ of a building. Accommodating a request by Assistant District Attorney Jennifer Hickman of Conley’s Homicide Unit, West Roxbury Municipal Court Judge Mary Ann Driscoll ordered Sanchez held without bail.

Hickman told the court that Castillo was attending a birthday party in her honor at a Willis Street residence on the night of Sept. 19 and into the early morning of Sept. 20.

During the course of that party, three men – including Sanchez – arrived and became engaged in a verbal altercation inside the residence. Another partygoer asked the three men to leave.

The three men did leave the building but continued to exchange words with other people outside. At about 2:30 a.m., Castillo leaned out a window and addressed them. It was at that time, prosecutors allege, that Sanchez fired multiple shots in her direction. One of those shots struck Castillo in the neck.

Castillo died of her injury within ten minutes of sustaining it, Hickman said.

Sanchez and his associates are said to have fled the scene in a red Honda Civic. A description of the car was relayed to Boston Police, who stopped it and brought the occupants in for questioning. A search warrant later executed on the vehicle led to the recovery of a firearm, prosecutors said.

In the days following the shooting, Boston Police interviewed multiple witnesses who gave significant information about Castillo’s death and its circumstances, Conley said.

“Those witness statements were critical in making the arrest,” Conley said. “It’s that kind of cooperation that helps us solve cases and prove them in court.”

Sanchez was represented by attorney Ronald Brandt. He will return to court on Oct. 22.