An official investigation has discovered that workers mending a rail-road barrier in India made improper connections in the network’s computerized signaling system, resulting in the country’s biggest rail accident in two decades.

The collision on June 2 at Bahanaga Bazar station in the eastern Indian state of Odisha killed 288 persons and wounded over 1,000 more.

The catastrophe occurred when a passenger train collided with a halted freight train, leaped off the tracks, and collided with another passenger train traveling in the other way.

In the probe report, seen by Reuters, the Commission of Railway Safety (CRS) investigators said the first collision occurred due to modifications done to the signaling circuit to fix frequent problems at a nearby rail-road barrier.

Local railway staff did not have a standard circuit diagram which led to a faulty connection in the signaling system when they tried to take the boom-barrier circuit offline for repair, it said. The malfunctioning system directed the passenger train onto the path of the freight train, it said.

Reuters last month reported for the first time that investigators were focusing on the repair work on the rail-road barrier and its possible connection to a manual bypass of the signaling system.

Indian Railways, the fourth largest train network in the world, is a state monopoly run by the Railway Board. The board reports to the Railways Ministry.

The rail network is undergoing a $30 billion transformation with gleaming new trains and modern stations under Prime Minister Narendra Modi’s push to boost infrastructure and connectivity but the crash raised questions about whether safety was getting enough attention.

The CRS probe report said there were lapses at multiple levels in the signal and telecom department and standard operating procedures were not followed during the repair work.