At least 17 people were killed at a high school in an affluent suburb in the south-eastern US state of Florida when a heavily armed man barged into campus and opened fire on terrified students and teachers.

The attack, at least the third school shooting in the last five months, ranks among the deadliest mass shootings in modern US history.

The 19-year-old suspect was a former student who had been expelled for disciplinary reasons.

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