Italian prosecutors are investigating seven olive oil producers for fraudulently selling ordinary olive oil as highly prized extra-virgin.

Prosecutors in the northern city of Turin announced the investigation Tuesday after customs officials independently confirmed the findings by the magazine "Il Test," which published a cover story last spring revealing that nine out of 20 bottles it tested were not extra-virgin as labeled.

The Italian farm lobby Coldiretti called on authorities to better protect Italy's olive oil sector, second only to Spain's with estimated annual revenues of 2 billion euros. Coldiretti said that the fraud was often perpetrated by adding imported olive oil to domestic, noting that Italy was the No. 1 olive oil importer in the world.