PARIS (AP) — Monaco's government says it's investigating how an alleged vast oil corruption scandal involving a Monaco-based company long went unnoticed, even though the principality has opened up its financial sector and cracked down on tax dodging.

Monaco police carried out several raids and detained executives of UNAOIL for questioning this week at the request of British anti-fraud authorities, he said. The company, reportedly suspected of paying huge bribes to secure contracts for multinational oil companies, denies wrongdoing.

First Minister Serge Telle told The Associated Press on Friday that the government is looking into the case in parallel to the judicial investigation. "It interests me personally as chief of the executive to know what allowed this to happen. There are probably lessons to learn."

Telle, who took up his position in January, called corruption a "plague that we must fight in all its forms," and said Monaco's days as a haven for dubious financial dealing "are over."

Authorities "are under constant instructions of the prince to open the doors and windows of the principality on these questions."

Despite years of rumors about wrongdoing by UNAOIL, Monaco authorities never investigated until Britain's Serious Fraud Office recently flagged the case.

Asked why not, Telle said, "All these things are dormant. That's the very principle of corruption — everything is very hidden until someone ends up revealing the facts."

He called the UNAOIL case "a big affair" — though one of many international financial scandals Monaco is investigating. "We are very relaxed and clear. The law applies and we will ensure it is respected," he said.

Citing an ongoing investigation, he wouldn't say whether any of UNAOIL's assets had been seized or whether the executives are free to leave Monaco while the probe continues.

One of the alleged corruption cases centers around the lucrative Kashagan oil field in Kazakhstan, according to a multi-part expose published Wednesday by the Huffington Post and Australia's Fairfax Media.

The report says the FBI and U.S. Justice Department are also involved in the investigation. They have not commented publicly on the case, nor has Britain's Serious Fraud Office.

Fairfax and Huffington Post say UNAOIL paid big bribes to secure contracts from Kazakh officials for foreign companies, notably Italy's Eni.

Eni said in a statement Friday that employees mentioned in the report have since left the company. It said the reported behavior "is in direct and obvious conflict with Eni's code of ethics." Milan prosecutors investigated the case in 2011-2012 but it's unclear whether anyone has been charged or jailed.

Another company named in the report is the offshore arm of Australian contract miner Leighton Holdings. On Friday, the Australian Federal Police confirmed they were investigating allegations that Leighton employees were involved in the payment of bribes during two oil projects in Iraq in 2010 and 2011. Leighton, which changed its name to CIMIC last year, would not comment.

A UNAOIL spokeswoman said the company "has no comment at this time."

Colleen Barry in Milan, Jill Lawless in London, Rod McGuirk in Canberra, Australia, and Raphael Satter in Paris contributed to this report.