LONDON—British police said Monday they were searching for a motive after a teenager allegedly threw a 6-year-old boy from a 10th-floor viewing gallery at London’s Tate Modern museum.

The boy was airlifted to hospital after the incident on Sunday afternoon. Police said Monday that his condition was critical but stable and his life was no longer in danger.

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

Nancy Barnfield 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?”

The younger boy had fallen from the open-air viewing platform to a fifth-floor roof.

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

Police say they don’t believe the suspect and victim knew each other.

“This was a truly shocking incident, and people will understandably be searching for answers,” said Detective Chief Superintendent John Massey of the Metropolitan Police. “At the moment, this is being treated as an isolated event with no distinct or apparent motive.”

Massey said police wanted to speak to anyone who “witnessed a male whose behavior seemed out of place, suspicious or worrying, in the hour or two before the incident in or near the gallery.”

Tate Modern, Britain’s leading gallery of modern art, sits on the south bank of the River Thames and was visited by almost 6 million people last year.

The 10th-floor terrace is part of a pyramid-shaped extension that opened in 2016 and offers panoramic views over London.

The gallery reopened Monday, but the viewing platform remained closed.

“Tate is working closely with the police to help with their investigations,” a spokeswoman said. “All our thoughts are with the child and his family.”

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