MBTA investigators have concluded that Monday's derailment of a Green Line trolley was caused by "human error."

T spokesperson Joe Pesaturo said in a statement that the operator "failed to operate his trolley in compliance with speed and signal requirements."

A source tells WBUR on Friday, the MBTA fired the operator, Sydely Gardner.

As we reported earlier this week, Gardner, was fired by the MBTA in 2010 after failing to report a collision with a pedestrian, but was reinstated by an arbitrator.

Gardner was operating a Riverside-bound Green Line trolley that crashed into a tunnel wall after derailing between Kenmore and Fenway stations Monday afternoon. Ten people suffered minor injuries in the incident.

Investigators had previously ruled out mechanical problems and issues with the track and signal systems.

Officials say a decision on Gardner's employment will be made in the next few days.

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