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Police in London said the teen picked up the child and threw him from the 10th floor of the Tate Modern gallery, CNN reported. The child was found on a fifth-floor roof, police said. He was in critical condition Sunday, CNN reported.
Police said that the teen was not known to the child, the BBC reported.
One visitor to the museum said she was on the gallery's 10th floor with her children when they saw people restraining the teen, whom she said appeared calm and did not fight back, CNN reported.
Nancy Barnfield said she had seen the teen earlier and said he was acting suspiciously. She said she had heard a loud bang as she walked through the gallery. Barnfield then said she heard the mother asking: "Where's my son? Where's my son?" the BBC reported.
The gallery was placed into lockdown as police investigated and all visitors were told to go to the museum's main hall, The Guardian reported. The visitors have since been allowed to leave.