WTC Building 7 (WTC 7) collapsed on the afternoon of September 11, 2001, at 5:20 PM, several hours after the North and South Towers fell. Here's a concise summary of the official account and the controversy:
Official Timeline of Events Leading to WTC 7 Collapse:

Proximity to Towers:
WTC 7 was located about 370 feet north of the North Tower (WTC 1).

Damage from WTC 1 Collapse (10:28 AM):
When WTC 1 collapsed, large debris struck WTC 7, causing significant structural damage to the south face and igniting fires on multiple floors.

Uncontrolled Fires:
Fires burned for approximately 7 hours on multiple floors. Due to the evacuation of the building and water supply issues (e.g., broken mains), automatic sprinklers failed, and the fires were not fought.

Collapse at 5:20 PM:
Video footage shows the building's penthouse falling first, followed by the entire structure collapsing symmetrically.

NIST Explanation (National Institute of Standards and Technology):

Cause:
The failure of a critical column (Column 79) due to thermal expansion of steel beams caused a progressive internal collapse.

Mechanism:
The heated floor beams expanded, pushed a girder off its seat, and caused the collapse of column 79, triggering a chain reaction.

Free-Fall Observation:
NIST acknowledged that for ~2.25 seconds, the building fell at gravitational acceleration (~9.8 m/s2) — but explained this as consistent with the global collapse mechanism once internal support was lost.