Somali-Norwegian Zakaria Bulhan, 19, pleaded guilty to manslaughter by diminished responsibility in the August 3 slaying of 64-year-old Darlene Horton.
He also pleaded guilty to five counts of wounding during a hearing Monday at London's Central Criminal Court.
Police subdued Bulhan with a stun gun after the late-night attack in Russell Square. The incident raised fears of terrorism, but police said it was not considered a terrorist attack.
Prosecutors said Bulhan was suffering an "acute" episode of paranoid schizophrenia at the time.
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