Police were questioning a suspect and working on Tuesday to understand what motivated a 17-year-old boy to attack the children, leaving two dead and six in a critical condition.
Swift said she was “completely in shock” and still taking in “the horror” of the event.
“These were just little kids at a dance class,” she wrote on Instagram on Tuesday.
“I am at a complete loss for how to ever convey my sympathies to these families.”
A total of nine children and two adults were hurt in the rampage in Southport in north-west England. Both adults and six of the children were in critical condition in local hospitals.
A 17-year-old boy was arrested on suspicion of attempted murder and was being questioned by police over the attack in Southport, north of Liverpool.
Local people left flowers and stuffed animals in tribute at a police cordon on the street lined with brick houses in the seaside resort near Liverpool â nicknamed “sunny Southport” â whose beach and pier attract vacationers from across northwest England.
Witnesses described scenes “from a horror movie” as bloodied children ran from the attack just before noon on Monday.
The suspect was arrested soon after on suspicion of murder and attempted murder. Police said he was born in Cardiff, Wales, and had lived for years in a village about five kilometres from Southport. He has not yet been charged.
Police said detectives are not treating Monday’s attack as terror-related and they are not looking for any other suspects.
“We believe the adults who were injured were bravely trying to protect the children who were being attacked,” Merseyside Police Chief Constable Serena Kennedy said.