The Office of Fair Trading raided offices of Mowlem last week only days after Carillion completed its agreed £313 million acquisition of the company.
The weekly Construction News reports this morning that OFT inspectors removed the computer of Mowlem's outgoing chief executive Simon Vivian, when they swooped on its head office in Isleworth, West London, which Carillion intends to close. Mr Vivian was visiting Carillion's Wolverhampton office at the time.
Carillion's finance director, Chris Girling, refused to confirm or deny yesterday that the computer, or any documents, had been taken, or to say whether police had accompanied the OFT inspectors.
"We don't know what it is all about," he said. Nothing revealed in Carillion's due diligence inspection of Mowlem's books before the bid would explain the move, he added.
The OFT is usually concerned with price-fixing issues than any question likely to have arisen from the acquisition.
Its inspectors also visited Mowlem's Nottingham office, part of its building division, which takes on contracts worth up to £15 million, many of them around £5 million.
One source suggested that it is easier to fix prices in contracts of this size than in complex civil engineering projects.
"They turned up at both offices en masse, which seems a bit heavy-handed," he added.
Mr Girling was presenting Carillion's 2005 results which included nothing from Mowlem, but £3.5 million pretax from the mechanical and electrical maintenance company PME acquired last March.
He said Mowlem would enhance Carillion's earnings materially in 2007 and have a "neutral" effect this year.
Carillion chairman Philip Rogerson added: "The acquisition of Mowlem will accelerate growth substantially and create significant value for shareholders."
Carillion expects to cut "hundreds" of jobs at Mowlem to help it hit annualised cost savings of £15 million.
Underlying profits adjusted to take account of the new international reporting standards were 15 per cent ahead at £55.5 million. A normal tax charge, in place of a low levy in 2004, trimmed the improvement in earnings per share to ten per cent.
A final dividend of 5.2p gives shareholders a seven per cent increase. The shares fell 8p to 327p, equivalent to 16 times the year's earnings, where they yield 2.4 per cent.
Carillion's revenues rose 15 per cent to £2.28 billion.
It finished the year with an order book 40 per cent higher than at the end of 2004 at £7 billion, not counting anything from Mowlem.