European Commission regulators have raided the offices of Star Alliance partners TAP Portugal and Brussels Airlines, in conjunction with inquiries into possible anti-competitive behaviour.
The unannounced inspections took place on 13 December, the Commission said.
Investigations into activities between the two carriers were opened earlier this year, along with similar probing of the Star pact between Lufthansa and Turkish Airlines. They focused on cross-selling of fares on the Brussels-Lisbon, Frankfurt-Istanbul and Munich-Istanbul routes.
"While the Commission was so far rather concerned about possible effects that codeshare agreements may have had for consumers, it now has, in addition, reasons to suspect illegal collusion between the parties," the Commission said.
"The Commission has concerns that the agreements may go further than the sale of seats on routes where the two companies are expected to compete."
National competition authorities joined the raids on the carriers' premises. The Commission points out, however, that the inspections are a preliminary step and do not necessarily mean the companies have engaged in anti-competitive behaviour.