World number one Daimler and number two Volvo confirmed they were both being investigated.
So too did Sweden's Scania and Germany's MAN, both allied to German auto giant Volkswagen, and Fiat Industrial, maker of Iveco trucks.
The EU move follows a British price-fixing probe in September last year. The UK's Office of Fair Trading contacted leading truckmakers including Scania, Volvo, Daimler and MAN as part of the investigation.
An OFT spokeswoman declined to comment on whether there was any link between this and Tuesday's EU raids.
"The European Commission can confirm that on 18 January 2011 Commission officials started to undertake unannounced inspections at the premises of companies active in the truck industry in several member states," it said in a statement.
"The Commission has reason to believe that the companies concerned may have violated EU antitrust rules that prohibit cartels and restrictive business practices and/or the abuse of a dominant market position."
Truckmakers have seen a revival in their fortunes since the worst of the financial crisis when orders plummeted. Much of the demand has come from developing markets.
The Commission did not say which companies it was investigating. It can fine companies up to 10% of their global turnover for infringing EU rules.
Heavy penalties
It has imposed more than €12 billion in fines against cartels in the last five years, with the highest-ever penalty of €1.38 billion levied on a group of car glass makers in 2008.
Such investigations typically take years and in a number of past cases were triggered by a whistleblowing company which was then let off without a fine. Penalties for the other infringers are generally heavy as the regulator seeks to deter such behaviour.
A spokesman for MAN said its offices had been searched and it had "assured the cartel authorities of its full cooperation".
Daimler said its Stuttgart headquarters had been searched and Fiat said its home base had also been raided, while Scania and Volvo said they were cooperating with the probe.
None of the truck makers gave any further details about the investigation.
MAN and Scania said in November they were in merger talks, as their main owner Volkswagen aims to cement full control of both and compete with world leader Daimler.
Volkswagen owns nearly 30% in MAN, 45.7% of Scania's capital and 71% of Scania votes.
MAN also holds a 17.4% voting stake and 13.4% of Scania's capital after a failed hostile takeover attempt.
(EurActiv with Reuters.)
