It was a draw-bar hook that punctured one of the gas tanks that derailed, cause a gas leakage in Hitrino village, Bulgarian authorities say.

The leakage and a lit-up stove subsequently triggered the explosion that killed seven people in the village of Hitrino, Transport Minister Ivaylo Moskovski has told lawmakers at a joint session of transport and internal affairs parliamentary committees.

Moskovski has confirmed earlier reports that the freight train carrying the cylinders moved at a speed twice the limit indicated by Hitrino railway authorities.

Passing through one of the railroad switches near Hitrino station (which is located in the village's central area), the cargo train had a speed of 80 km/h, while the limit was 40 km/h, the Bulgarian National Radio quotes Moskovski as saying.

Of the twelve gas tanks carried by the locomotive, five made it through the switch, but the others went off the tracks. The hook pulling the ninth wagon then pierced a cylinder, according to the minister.

On Monday, two train drivers were indicted over the tragic incident.

However, more people are expected to face additional charges.

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