The raid left Controlled Demolition managing director Darren Palin bemused. He said: 'We were very surprised when they turned up but gave them full access to whatever they wanted.
'It is always a concern when something like this happens but we don't expect anything to come of it.
'We are not guilty of anything and I can only assume that they are looking at demolition as part of their construction crackdown and we happen to be one of the highest profile firms in the sector.'
OFT officials have highlighted construction as one of five priority areas it will be concentrating resources on this year.
An OFT spokeswoman confirmed the raids and added: 'We can't say anything more. Just because we have visited firms you can't assume guilt.'
Terry Brown, managing director of Brown & Mason, said: 'We have got nothing to hide. As far as I know they didn't find anything and they went away.
'They said we might never hear from them again. They looked around and we helped them.'
The six-strong team of investigators spent all day at the firm's offices.
Mr Brown said the raids could damage some firms' businesses and blamed an aggrieved rival for the tip-offs.
He added: 'This kind of thing can be very dangerous when we haven't done anything wrong. I think it's somebody putting the poison in, I really do. Maybe they can't get on a tender list and they turn around and say 'it's collusion'.'
A source at KDC said: 'I don't know why they visited us, a lot of our work is negotiated. We let them go through our records and they didn't take anything away.'