Poland has hit out at an “incomprehensible” decision by the European Commission to freeze aid worth billions for Polish road building programs over a suspected price-fixing scandal.

The European Commission froze funds worth PLN 3.5 billion for three projects managed by Poland’s highways agency, the GDDKiA, after Polish prosecutors charged 11 people with attempting to create a cartel.

The Commission said those charged are 10 current and former managers for large construction firms and one highways agency director.

On Wednesday, the Commission said it had decided to freeze another 4 billion euros in aid, demanding that Poland check all road projects managed by its highways agency and jointly financed by the EU.

Poland’s opposition parties called on the country’s transport minister, Slawomir Nowak, to resign. Adam Hofman, spokesman for the opposition Law and Justice (PiS) party, demanded an explanation from the government. “What happened to public money? Whose palm was greased,” Hofman asked.

But Poland’s Regional Development Minister Elzbieta Bienkowska said the European Commission’s decision was “incomprehensible” and “curious.” She insisted that the case highlighted the efficiency of the Polish authorities in cracking down on suspected abuses.

“Poland is the injured party in this case, because it is Polish investigators who discovered that there may have been – in two or three cases – a price-fixing deal by contractors,” Bienkowska told the Polish Radio public broadcaster.

Bienkowska and Nowak are set to discuss the case with the European Commission on Friday.