European Union and Spanish antitrust officials have carried out surprise inspections at the premises of companies producing cement and related building products in Spain, the European Commission and Spain's National Competition Commission, or CNC, said Wednesday.

The E.U. and Spanish probes are separate, though they are targeting some of the same companies on suspicion of taking part in price-fixing cartels, a CNC spokeswoman said. The E.U.'s inspections are the first step in an E.U. antitrust investigation.

The CNC said its inspections represent a preliminary step and don't necessarily mean the beginning of a formal proceeding. In the event that illegal practices are proved, the CNC could impose penalties of up to 10% of the total sales volumes of any companies involved.

Cement giant Cemex SA (CX) said in a filing with U.S. regulators that Spanish antitrust authorities searched the offices of its unit in Spain.

Cementos Portland Valderrivas SA, which is controlled by construction company Fomento de Construcciones y Contratas SA (FCC.MC), "is one of many companies under investigation and it is cooperating" with authorities, said a company spokesman.

Swiss cement company Holcim Ltd. (HOLN.VX) confirmed its offices in Spain have been searched in the context of the E.U. probe, adding it is fully cooperating with authorities.

Officials at local producer Cementos Molins weren't immediately available for comment.

-By Carolyn Henson, Dow Jones Newswires; +32 2 741 1481; carolyn.henson@dowjones.com (Martin Gelnar in Zurich and Pablo Dominguez in Madrid contributed to the article.)