Lizzie,

From Royston's last post above:

WTC 7 was another structure supported largely by its steelwork. It was on fire from the early morning until it collapsed later that day. It was hit by the collapsing North Tower. It had a deficient sprinkler system. Firefighters were in the building throughout the day and reported the spreading fire, lack of water pressure and structural damage from falling debris. at 14:00, fireighters noticed the SW corner of the building was bowing outwards and they evacuated it at 15:30. It collapsed at about 17:20.

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I've highlighted the relevant bits to make it easier for you