Bulgaria’s national security agency and prosecutors raided the Agriculture Ministry March 18 and seized documents in an investigation into large-scale illegal land deals.

The raid was part of Bulgaria’s drive to assure the European Union it is serious about fighting chronic corruption and organized crime. Brussels has punished Sofia for failing to produce results by freezing large amounts of EU aid.

Prosecutors are investigating swaps of thousands of hectares of state-owned arable land and forests for private land that took place this and last year, the Sofia City Prosecutor Nikolai Kokinov said, without giving further details.

Land swaps have become a major source of corruption in the EU newcomer in the past several years because a real estate boom made state and municipally owned land on the Black Sea coast and in mountains extremely valuable.

“We have been informed by the chief prosecutor’s office, which got signals from the National Security Agency, that officials at the Agriculture Ministry have allegedly made illegal swaps of land,” Kokinov said.

“When we collect enough evidence about concrete people, we will raise charges,” he said.

The Agriculture Ministry declined comment, saying it would issue a statement later in the day.

Bulgaria has lost eight billion levs ($5.33 billion) in the past four years due to swaps of high-quality, state-owned land for cheaper, fragmented private land, according to a report by an anti-corruption coalition of 30 non-government organizations.