Friends of the teenager arrested over the murder of a US tourist in the Russell Square stabbing attack have spoken of their shock.
One former schoolmate said Zakaria Bulhan was a "nice, quiet kid".
Bulhan, 19, a Norwegian national of Somali origin who moved to the UK in 2002, remains in police custody after the knife rampage on Wednesday night in central London, which left US citizen Darlene Horton, 64, dead and five others injured.
Ms Horton was the wife of a university professor from Florida and was just hours away from flying home.
She was visiting the capital with husband Richard Wagner, who was teaching summer classes, when she was attacked.
Florida State University said the couple had planned to return to their home in Tallahassee on Thursday.
University president John Thrasher said: "There are no words to express our heartache over this terrible tragedy.
"We are shocked that such senseless violence has touched our own FSU family and we will do all we can to assist Professor Wagner and his loved ones, as well as his friends and colleagues in the psychology department, as they mourn."
Scotland Yard has said no evidence has been found of radicalisation or anything to suggest the attack was "in any way motivated by terrorism".
The schoolmate, who went to Graveney School in Tooting, south London, with Bulhan and wished to remain anonymous, said: "It's one of the most shocking things I have ever heard.
"No-one would have expected that he would turn out like that.
"He was a quiet, nice kid who had banter with people. He was just a nice kid.
"I have seen some reports that say he was bullied but it was just banter - he gave back as good as he got. It was a two-way thing."
A British man who suffered a stab wound to his stomach in the attack remains in hospital in a "serious but stable" condition while an American man, an Australian man and Australian woman, and an Israeli woman were all discharged from hospital on Thursday after receiving treatment for their injuries.
The Israeli teenager who was injured in the attack has said she does not know whether to call it "luck or fate" that she escaped death.
Yovel Lewkowski, 18, who was visiting London from Tel Aviv and was stabbed in the upper arm, said she watched as Ms Horton "perished in front of my eyes".
In a moving Facebook post, she wrote: "I'm not afraid, I want to keep going with the trip.
If you can't see the Facebook post above, click here.
"The last few hours have taught me to appreciate every second, I don't know whether to call it luck or fate, but no doubt it is some sort of miracle compared to the woman who stood right next to me and perished in front of my eyes: a matter of seconds, metres, breaths.
"I send strength to the family and ask that you take advantage of every moment - nothing is to be taken for granted.
"It's time to say thank you to the wonderful people in your lives... I thank everyone for their warm wishes."