A 15-year-old girl has pleaded guilty to second-degree assault in a Seattle bus tunnel beating of another girl that was captured on surveillance video.

The Seattle girl who entered her plea Tuesday had been charged with first-degree robbery in the Jan. 28 attack. However, the King County prosecutor's office says there wasn't enough evidence to prove that count.

Friends and relatives of six people arrested in the attack say the 15-year-old defendant and the now-16-year-old victim had a long-standing dispute. Two other juveniles have pleaded guilty to fourth-degree assault. Three adults await trial on robbery counts.

The widely viewed video showed uniformed security guards doing nothing to stop the beating. The attack prompted changes in bus tunnel security, including allowing security guards to intervene in tunnel fights rather than merely "observe and report."