Two men, aged 25 and 27, have been arrested after a flare hit a linesman during the Premier League match between Aston Villa and Tottenham Hotspur at Villa Park, West Midlands Police have said.
Television replays showed a missile being fired from the away section after Andros Townsend's first-half opening goal in Spurs' 2-0 away win.
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Authorities have warned that the death toll is likely to rise as much of the building is yet to be searched.
The PM will sound out the possibility of new free trade agreements with Gulf states for once the UK has left the EU.
In the unexpected announcement on Monday, Mr Key said it was the "right time" to leave politics.