Russian leader Vladmir Putin has called the shootings in a school in Connecticut "particularly tragic" as the majority of the 28 killed were children.
The Kremlin said in a statement:
Vladimir Putin asked Barack Obama to convey words of support and sympathy to the families and friends of the victims and expressed his empathy with the American people.
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Broadcaster PBS halted distribution of his nightly interview show and CBS News immediately suspended the 75-year-old.
Twenty years ago I reported from Mostar at the end of the Bosnian War. I returned to be reunited with some of the children I filmed then.
The alarm was raised shortly before 2am on Tuesday. Some 20 people reportedly exited the building before crews arrived.