Emergency teams are to begin draining the dangerous materials out of several tanks that derailed in Hitrino in northeastern Bulgaria on Saturday morning.

Seven people died after one of the cisterns exploded. Twenty-nine were injured, while dozens of families lost their home after the blast left buildings in a 300m perimeter completely ravaged.

Firefighters and other emergency staff spent the night pouring water on the tanks to reduce the risk of any fire or blast during the drainage operation, public broadcaster BNT says.

It will take more than 24 hours to take all the liquefied petroleum gas (LPG) and propylene out of the cisterns in a safe way.

Biser Minchev, chief inspector at the National Railway Infrastructure Company (NRIC), says it is not yet clear what caused the development.

While the case is left up to the investigative service and the prosecutor's office, the NRIC will be "carrying out its own probe", Minchev has told the BNT.

There was a rupture in the rail composition right before the incident, he has confirmed, but has refrained from commenting versions that speeding or a contact of waggons with electricity lines caused the explosion.

"We cannot say for now at what speed the train was moving when it entered the station," he has explained. The recording devices of the train are yet to be analyzed and give precise data.

"To me this is the heaviest [train] crash to occur in Bulgaria," he has added.

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