When a friend planned a birthday party for 18-year-old Paula Castillo, a Suffolk County prosecutor told a Superior Court jury in an opening statement this morning, “it wasn’t supposed to end with the death of a young woman.”

Assistant District Attorney Jennifer Hickman told the court that, on the evening of Sept. 19, 2009, three men walked into that party at 28 Hillis Road – RUBEN SANCHEZ (D.O.B. 9/6/90) of Hyde Park, MIGUEL VASQUEZ (D.O.B. 5/5/91) of Boston’s South End and ANDRES BREA (D.O.B. 1/25/90) of Roxbury – and by approximately 3 a.m., Castillo was dead from a gunshot wound to the neck.

Sanchez is charged with second-degree murder in connection with Castillo’s Sept. 20 shooting death. Vasquez is charged with assault by means of a dangerous weapon, and Brea is charged with being an accessory after the fact to assault by means of a dangerous weapon, for their roles during or in the aftermath of the shooting.

All three defendants are additionally charged with unlawful possession of a firearm, unlawful possession of ammunition, and unlawful possession of a loaded firearm.

Prior to the shooting, Hickman told the court, Vasquez became involved in a verbal argument with the hostess of the party. “You’ll hear that it was a verbal argument, it wasn’t physical, and there weren’t weapons involved,” She said. Other people at the party intervened in the argument and attempted to calm the situation, she said, but as a result of that argument, the hostess “made the decision to tell the three defendants to get out of the apartment.”

The three defendants complied, but they continued to exchange words as they went outside. They walked out to Brea’s red Honda Civic and that’s when “Mr. Sanchez pulled out a revolver – and with Mr. Vasquez standing next to him, giving him encouragement – began shooting in the direction of 28 Hillis Road,” Hickman said.

At about this time, an off-duty Boston Police officer who lived nearby looked out the window of his home, Hickman told the court, and observed a crowd outside of the Hillis Road home along with a red, two-door vehicle on the street, and “a male holding a gun, shooting in the direction of 28 Hillis Road.”

Hickman said that the off-duty officer reported the shooting to the main desk at the District E-18 police station, believing that officers would be able to respond more quickly from that location than if he called 911. The officer further observed “the shooter – Mr. Sanchez – get into that vehicle, observed the vehicle drive a short distance, make a U-turn, and stop in front of 28 Hillis Road for one final round let off from the front seat passenger,” the prosecutor said.

What that witness was unable to observe, Hickman said, was Castillo hanging out of the window when the shots were fired. He “didn’t see Ms. Castillo getting struck by a bullet. He did not observe her run down the stairs with blood coming out of her neck, blood pouring out of her mouth, unable to speak,” she said.

Castillo collapsed in the kitchen, and was surrounded by friends of hers who were screaming and crying, Hickman said. Some partygoers began to administer CPR to the victim “trying to keep her alive,” she said.

When Boston Police officers responded to the scene, Hickman said, they observed blood in the hallway, on the stairs and on the walls. Castillo had been struck in the neck by a bullet, and was “lying on the ground in a pool of blood.”

Even though officers “were dealing with people who had just witnessed a friend who was shot, and died,” Hickman said, “they were able to get important information” from those witnesses. Some of the witnesses were familiar with at least two of the defendants and were able to provide descriptions to police. That information, together with a description of the motor vehicle that fled the scene, and a rapid police response, led to police officers locating and stopping a car within minutes of the shooting that matched the descriptions provided by witnesses. After being pulled over and questioned by police, the defendants allegedly gave different alibis.

In addition to that evidence, Hickman told jurors, “you’ll hear … that Mr. Sanchez tested positive for gunshot residue. You’ll also hear that the vehicle that the three defendants were in also tested positive for gunshot residue.”

In the trunk of the car, she continued, officers found a revolver.

“The revolver had five rounds, four of which had been discharged, and there was still one live bullet in the chamber,” she said.

Kara Hayes, Chief of Conley’s Victim Witness Assistance Program, is the advocate assigned to the case. Sanchez is represented by attorneys Michael Doolin and William Keefe; Vasquez is represented by attorney Daniel Solomon; and Brea is represented by attorney Eleftherios Travayiakis. Testimony is ongoing before Judge Raymond Brassard in courtroom 808.