A 15-strong team from the National Investigative Service is to have access to the area where an explosion demolished an ammunition factory in Bulgaria's north-west, killing 15 people.

This is the first time anyone, except for a fire-brigade unit using special equipment, will enter the site after the 24-hour ban was lifted on Thursday, 21:00 EEST (19:00 GMT).

Also on Thursday evening, the prosecuting announced it was opening pre-trial proceedings over the incident. Chief Prosecutor Sotir Tsatsarov had earlier announced he would personally be in charge of coordinating the investigation.

Tens of documents seized at the offices of Videx JSC, the company owning the Midzhur plant, are now to be carefully examined.

It has so far been established that employees at the Midzhur plant, located near the village of Gorni Lom, were currently working on a contract with a Greek company to deactivate land mines at the time when an unspecified amount of explosives went off.

Under the agreement, more than 600 000 mines were received between July 8, 2008 and July 31, 2013.

Some 900 000 more were yet to be deactivated as of Wednesday, when the blasts occurred.

Marin Ivanov, who formerly managed the Terem military plant, told private national channel NOVA TV that Greek mines were difficult to handle, required expensive technology and carried a high risk.

The daily Monitor quotes retired Col Ivan Boyadzhiev as saying Bulgarians were not quite familiar with their technology.

2010's incident at the Midzhur plant also involved Greek mines.

Valeri Mitkov, the factory's owner, is to be interrogated at the National Investigative Service.

It is not yet known what caused the explosions, with authorities and most experts attributing them to a human error and not to working conditions and violations registered earlier.

A hypothesis raised by a trade-union leader and later reiterated by some experts, suggests a new type of product was being tested at the time of the blast without any previous instructions.

Friday is a day of national mourning, and most parties have declared they will put off their last campaigning events before the snap general vote on October 5.

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