The police said Thursday that they had arrested a second suspect in connection with the gang-rape and murder of a teenager who was mutilated and left to die at a building construction site last week in a case that angered the nation. The 17-year-old woman in the Western Cape lived long enough to identify one of her attackers, a 22-year-old. The police arrested him, and on Thursday they said they had arrested a second suspect, a 21-year-old. They promised more arrests soon, though it is not known how many attackers were involved.