Two BART workers were killed on a section of track between the Walnut Creek and Pleasant Hill stations about 1:53 p.m. on Saturday.

The workers were performing track inspections when they were hit by a northbound train and died at the scene, according to Jim Southworth, investigator-in-charge for the National Transportation Safety Board, which is now heading the investigation. The four-car train, which was on a routine maintenance run, was in automatic mode under computer control at the time of the accident, though an “experienced controller” was at the controls, according to a statement released by BART.

The two deceased, whose names have not been disclosed, were a BART employee and a contractor. Both had extensive experience working around trains in both the freight train and the rapid transit industries, according to the statement. The procedures involved in track maintenance require one employee to inspect the track and the other to act as a lookout and notify of any oncoming traffic.

According to BART Assistant General Manager Paul Oversier, workers performing such duties with “simple approval” must be able to clear the operating space within 15 seconds of a train approaching their location.

There were six people on the train at the time of the incident, according to audio transmission from BART track and yard operations.

The BART employee who was killed was a union member and chose to cross the picket line in spite of the ongoing strike, AFSCME Local 3993 President Patricia Schuchardt told ABC7 News.

Managers and former train operators have been moving trains during the strike, according to the Contra Costa Times. BART police did not confirm who was driving the train at the time of the accident.

The NTSB arrived on scene at 12:40 p.m. on Sunday.

During a press conference, Southworth said the NTSB will conduct its investigation for approximately four to 10 days. Southworth told reporters that there was an inward-facing camera inside the cab, but there was no exterior camera that recorded footage of the accident.

The NTSB will analyze all mechanical equipment and tracks involved in the crash. After inspections and interviews with those on the train are complete, the NTSB will make safety recommendations to BART to prevent further accidents of this nature.

BART workers have been on strike since Friday after a week of negotiations between BART unions and management failed to produce a contract the two sides could agree on.