The investigation, which is directed by the specialised prosecution department, is targeting alleged irregularities in the approval of loans, involving the official who at the time of the deed was in charge of one of the regional units of the bank, according to a release from the General Police Administration.

While the police release referred to a major Slovenian bank without specifying, NLB has confirmed the searches are related to Slovenia's largest bank.

"The investigation was requested by NLB due the existence of reasons to suspect crime", the bank said in a statement, adding that it could not comment on details in the interest of ongoing investigation.

Twenty NBI investigators and several police officers are involved in the six house searches on private and business premises, the police said.

A 58-year-old male who was responsible at a regional unit of NLB and another 57-year-old male from Ljubljana have been detained for questioning, according to the police.

Today's operation wraps up the first phase of police investigation as part of which NBI investigators have been cooperating with foreign law enforcement authorities.

The suspected acts under investigation include abuse of office or trust in corporate activity, which carries a custodial sentence of up to eight years.

The daily Delo reported online that the premises of the NLB office in Šmartinska street and several private homes were raided early on Thursday.

According to business daily Finance, the department at Šmartinska is responsible for the most problematic borrowers.

The daily notes that NLB is heavily exposed to bankrupt builders SCT and Vegrad, citing unofficial information that one of the things investigators are looking into is contentious financing of Celovški dvori, a residential area in the Ljubljana Šiška borough built by Vegrad.