Bulgarian police have arrested the owner of the safety vault in downtown Sofia, which was robbed in the wee hours Monday.

According to the BTA news agency, citing unnamed sources, Borislav Ralchev is investigated by the Prosecutor's Office for illegal bank activities. The prosecutor is expected to ask for his permanent detention.

The investigators have collected so far police reports and testimonies from the victims of the robbery, saying they gave Ralchev money for safekeeping against high interest rates and dividends and without receiving invoices.

Pre-trial procedures in the robbery case have already been launched, They are against an unknown perpetrator for large-scale theft.

Earlier Tuesday, Interior Minister, Tsvetan Tsvetanov, announced the police is probing a possible pyramid scam, saying it was alarming customers of the safety vault have not been receiving interest for an entire year.