The European Union said Thursday it suspected major truck makers of operating an illegal cartel to rig prices, saying they faced heavy penalties if found guilty.
Competition Commissioner Margrethe Vestager said the companies had been formally notified they were under suspicion of breaching anti-trust rules, but did not name them.
Germany's MAN and Daimler Benz, as well as Sweden's Volvo confirmed they were involved in the probe.
The investigation is based on raids carried out on large truck manufacturers in 2011, which also involved Sweden's Scania and Italy's Iveco.
"It seems that heavy-weight and medium-weight truck companies have agreed to coordinate pricing in European economic area," Vestager told a news briefing.
"If the case is proven, this would be a very serious infringement," she said.
The decision to send a so-called "statement of objections" to the companies, while signifying deep suspicion of wrongdoing, does not presume guilt in the matter.
Busting cartels in the EU can be a long process often taking years to snake through the European courts.
If confirmed, companies face a fine of up to10 percent of annual worldwide sales.
Vestager said that the suspected cartel involved many companies, with activity peaking about ten years ago.
Asked if settlement were possible in the case, Vestager said that given the gravity of the infractions, "settlement will be difficult here".
In a statement, the Volvo Group said it was "cooperating fully with the authorities" and had already warned shareholders of a probable hit on profit.