The European Union's antitrust regulators are investigating a suspected cartel among a number of producers of steel wire and rods, known as prestressing steel.
The European Commission said in a statement on Thursday it suspected the companies of "participation in a cartel in violation of EU rules on restrictive business practices."
It did not name any of the suspected companies, to which it sent last October a statement of objection, a formal step in Commission antitrust investigations.
The Commission said it was now considering their replies. If found guilty, they may face a fine of up to 10 percent of their turnover.