The Office of Fair Trading has raided the offices of a subsidiary of the construction group Morgan Sindall.
The trading regulator targeted Bluestone, which is composed of a network of regional construction businesses, last week.
A spokesperson for Morgan Sindall said: "We can confirm that Bluestone, one of our subsidiaries, is co-operating fully with a routine investigation by the OFT and will continue to do so.
"We are unable to make any further comment at this time."
In its results presentation earlier this month, Carillion revealed that OFT officials had visited two Mowlem offices.
Chris Girling, Carillion finance director, said: "We don't know what it's all about. We are waiting for the OFT to report back; it was a surprise to us."
The OFT declined to comment on the continuing investigation.