"The horror of yesterday's attack in Southport is still going through my mind and I'm in complete shock," the US singer wrote in her Instagram story.
The attack in the north-west English town took place at a Taylor Swift-themed vacation camp for primary school children.
The attack has claimed a total of three lives so far. Two children were stabbed to death and another was so seriously injured that he died in hospital. At least five other children are still in a critical condition, as are two adults who tried to help and were attacked.
The motive is unclear. A 17-year-old is suspected of the crime and is being questioned by investigators. According to an eyewitness, he is said to have driven up in a cab and refused to pay the fare.
"They were just little kids at a dance class," Swift continued. "I don't know how to ever express my sympathy to these families." The singer expressed her sorrow at "the loss of life and innocence" and recalled the "horrific trauma" of the relatives and first responders.
Home Secretary at the scene of the crime
The vacation course for children aged between six and eleven was fully booked with 25 participants, as reported by the BBC. The plan included a dance and yoga workshop as well as making bracelets like those worn by many Taylor Swift fans.
The investigation continued in the city between Liverpool and Blackpool. The crime scene was cordoned off over a wide area. Mourners laid flowers and cuddly toys. Home Secretary Yvette Cooper visited the scene to gain an impression. The local football club Southport FC canceled a test match.