A British teen pleaded guilty Monday to murdering three girls and attempting to kill 10 other people in what a prosecutor said was a “meticulously planned” stabbing rampage at a Taylor Swift-themed dance class in England last summer.

The teen arrested in the aftermath goes on trial Monday on three counts of murder, 10 counts of attempted murder and additional charges related to possessing the poison ricin and for having an al-Qaida manual.

Axel Rudakubana, 18, changed his pleas from not guilty to guilty on what was due to be the first day of his trial at Liverpool Crown Court.