A British man has admitted to murdering 3 girls and trying to kill 10 others at a Taylor Swift-themed dance party in the UK last year.

18-year-old Axel Rudakubana shocked the court on day one of his trial at Liverpool Crown Court Monday by pleading guilty to murdering Alice Dasilva Aguiar, 9, Elsie Dot Stancombe, 7, and Bebe King, 6. He’s facing life behind bars when he's sentenced Thursday.

Axel, who was 17 at the time of the attack, also pled guilty to 10 counts of attempted murder and charges for having ricin and an Al-Qaeda manual linked to the July 29 attack in Southport, UK, last year.

He had consistently stayed mum through all his court appearances ... until he finally said "guilty" to each of the 16 charges. Rudakubana had previously pled not guilty in the trial, and Monday's plea was a surprise as the trial began.

The attack wasn’t labeled a terrorist act since the motive was unclear -- and prosecutors say they still don’t know why he did it.

The attack left people around the world horrified, with Taylor herself saying she was "completely in shock" over the murders.