A 19-year-old Rockville woman pleaded guilty to second-degree murder and 14 other charges yesterday in a Gaithersburg home invasion during which one teenager was fatally shot and another was stabbed. She is the third teenager to plead guilty in the case.

Montgomery County prosecutors said Chelsa Sommer, of the 700 block of Woodburn Road, waited outside the home in the 8100 block of Brucar Court while two friends armed with a shotgun, handgun and knife entered through an unlocked door. Sommer and her friends planned to rob the residents because they had bought some fake cocaine there, prosecutors said.

After confronting five teenagers in the home and binding their wrists and ankles with duct tape, prosecutors said, Sommer's friends stabbed one teenager and shot Ali Rabonik, 18, in the face when the shotgun fired during a struggle. Sommer was not in the home, but she was legally responsible for the robbery and murder because she acted as the lookout and getaway driver, said Assistant State's Attorney Robert Hill.

Maryland sentencing guidelines in the case call for 65 to 115 years in prison, Hill said. The two other teenagers, Shiva Dayani and Joshua Friedman, pleaded guilty to the same charges. All three are to be sentenced in the fall.