The Metropolitan Police said in a news release the 6-year-old boy was thrown from the tenth-floor viewing platform, landing on a fifth-floor roof before he was transported to a hospital by London’s Air Ambulance. As of Monday afternoon, police said the child’s condition was critical but stable and his life was no longer in danger. He remains at a hospital with his family, who are French nationals visiting London.

“This was a truly shocking incident, and people will understandably be searching for answers,” senior investigating officer DCI John Massey said in a statement.

A 17-year-old male is held in suspicion of attempted murder over the incident, which happened just while the gallery was packed with visitors.

“We are grateful for the support of the public, some of whom detained the male arrested in the immediate aftermath of the incident,” Massey said. “He was arrested by officers very quickly afterward.”

Nancy Barnfield told the Associated Press she was at the gallery with her family when she heard a “loud bang,” and then saw a woman screaming “Where’s my son, where’s my son?”

Barnfield said that a man on the platform was restrained by other visitors until police arrived. She said he “just stood there and was quite calm.”

Authorities said they don’t believe the suspect and victim knew each other, and the incident was “being treated as an isolated event with no distinct or apparent motive.”