Trial Begins in Shooting Death of Tayshana Murphy In the final moments of her young life, she danced in the playground next to her apartment building with her big sister and friends, joyfully oblivious to the bloody turf fights roiling from block to block all around her. Moments later, prosecutors say, everyone scattered when two combatants in that turf war walked onto the playground in Upper Manhattan determined to take a life, any life, as retribution for a beating earlier that night. They found the young woman, Tayshana Murphy, on the fourth floor, nervously waiting for an elevator to the safety of her apartment, where she lived with her mother and brothers. "I'm a girl!" she shouted in a panic as two men climbed the stairs behind her, according to a prosecutor. I'm not with them. One of the men shouted that he did not care, walked up the stairs, and put three 9-millimeter bullets in Ms. Murphy, an 18-year-old high school basketball star with dreams of playing in the W.N.B.A. She died on the hallway floor on Sept. In a trial that began on Thursday in State Supreme Court in Manhattan, prosecutors described Ms. Murphy as a victim of a pointless rivalry between young men at the Grant Houses and the Manhattanville Houses, two public housing projects in Morningside Heights. Tiana Walton, an assistant district attorney, told jurors that Ms. Murphy and the two defendants - Tyshawn Brockington and Terique Collins - lived "in a world that few of us see," a world of "grievances, fights and wars." "It was in this world that Grant and Manhattanville were not neighbors; they were enemies," Ms. Walton said. Ms. Walton said either Mr. Brockington, 23, or another man who is awaiting trial, Robert Cartagena, 22, fired the shots that killed Ms. Murphy. Both are charged with murder and face up to life in prison. Mr. Collins, 25, who is on trial with Mr. Brockington, is charged with criminal weapon possession and faces up to 15 years in prison on that charge. Ms. Walton said fights between the young men from the two projects went on throughout the night of Sept. 10, 2011, and into the early morning hours of the next day. Bats and bottles were used as weapons, and people were chased up and down Old Broadway, a small street near the two projects. Mr. Brockington was beat with a cane at some point, Ms. Walton said. Then, around 3 a.m., Mr. Cartagena and his girlfriend were chased down 125th Street and attacked. Mr. Cartagena and Mr. Brockington crossed paths soon after and hatched a plan for revenge. "They didn't want just bats," Ms. Walton said. They wanted something more, something illegal. She said they contacted Mr. Collins, who kept a handgun they all shared. They retrieved the gun and went to the Grant Houses to "smoke someone." Lawyers for Mr. Brockington and Mr. Collins said both men were innocent. They said prosecutors wrongly focused their attention on the two based on rumors. "There are many people other than Tyshawn Brockington with reasons to be hostile to the young men of Grant Houses," his lawyer, Daniel S. Parker, said. Ms. Murphy played basketball for Murry Bergtraum High School for Business Careers in Manhattan. She was ranked on an ESPN Web site as the nation's 16th-best point guard in her class. Jurors were shown a cellphone video of Ms. Murphy dancing with her sister, Tanasia Williams, and other friends as a boombox played that night. Ms. Williams, 23, said she had gone upstairs earlier than her younger sister. She was awakened by their brother screaming. She ran downstairs to find their mother cradling Ms. Murphy's lifeless body. "My sister was laying there, blood all around her head," Ms. Williams said, choking back tears. My mom was screaming and crying, holding her head, trying to pick her up.