Briefly-
On Sept. 11, 2001, 7 WTC was damaged by debris when the nearby North Tower of the WTC collapsed. The debris ignited fires, which continued to burn throughout the afternoon on lower floors of the building. The building's internal fire suppression system lacked water pressure to fight the fires, and the building collapsed completely at 5:21 pm. The collapse began when a critical column on the 13th floor buckled and triggered structural failure throught, which was first visible from the exterior with the crumbling of the east mechanical penthouse at 5:20 pm.
Above from Wikipedia, but also can be found by searching newspaper archives.
"As the North Tower collapsed....heavy debris hit 7 WTC causing damage to the south face....a large vertical gash on the center-bottom corner from the 8th to 18th floors and other damage as high as the 18th floor.
The sprinkler system was not fully automatic but required manual initiation of the electrical fire pumps...water pressure was low with little or no water to feed the sprinklers.
During the afternoon, fires were seen on floors 6-10, 11-14, 19-22 and 29-30.
A bulge was noted by firefighters between the 10 and 13th floors, a sign that the building was unstable and might collapse. About 3:30 FDNY Chief Nigro decided to evacuate the area. No casualties were associated with the collapse.
FEMA investigation- Fires on multiple stories exposed structural elements to high temperatures for a sufficient period of time to reduce their strength to the point of collapse.
Further investigations by NIST expanded and somewhat altered the FEMA report, but the idea of thermite or other explosive was dismissed.

The Secret Service and SEC lost unbacked evidence, as noted in Lizzie's post.