May be from huge balloons full of gas according to this report, but this is just one initial possibility reported.

Explosions at Manchester were cause by huge balloons filled with gas, there are multiple fatalities #ManchesterArena pic.twitter.com/XILprKH9p8

Update:

Via Daily Mail:

Another witness Jenny Brewster told MailOnline: ‘We were exiting the building when it happened. We’d headed towards the main doors as Ariana was performing the last song because we wanted to beat the crowds, but – as we made our way there – a wall of security men blocked it and told us to go the other way.

‘Seconds later they shouted ‘RUN!’ and the explosion happened right behind them. Hundreds of people were running and screaming. Those men saved our lives.’

Catherine Macfarlane told Reuters: ‘We were making our way out and when we were right by the door there was a massive explosion and everybody was screaming.

‘It was a huge explosion – you could feel it in your chest. It was chaotic. Everybody was running and screaming and just trying to get out of the area.’

One witness wrote on Twitter: ‘Just ran from an explosion, genuinely thought we were gonna die.’

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