[1]: 36 At 6:18 a.m., the J train was running at full speed along the Williamsburg Bridge's Brooklyn approach.

Immediately afterward, the National Transportation Safety Board (NTSB) announced that it would investigate the incident.

Gibson's blood had tested negative for drugs and alcohol,[3] and the NTSB found that there were no other major distractions at the time, including loud noise from passengers.

However, on the day of the crash, the rods failed to align, and so the emergency brakes on the J train were not engaged.

[6] The New York City Transit Authority (NYCTA), the division of the Metropolitan Transportation Authority (MTA) that operated the subway system, conducted its own investigation and concluded that the Williamsburg Bridge signals were spaced too closely together.

Trains' maximum speeds on straight track segments were throttled from 55 miles per hour (89 km/h) to 40 miles per hour (64 km/h), and the MTA installed "grade-time" signals around the system to ensure that a train could only travel under a certain maximum speed before it was allowed to proceed.

This modification of signals led to increases in train delays around the system, which in turn was a contributing factor to a transit crisis in 2017-2018.