The US state of Massachusetts is investigating whether the software giant Microsoft has violated its anti-trust settlement with 18 other states.
Massachusetts said it would look at allegations that the company retaliated against a computer maker for promoting the rival operating system, Linux.
In a court filing, the state said that none of the allegations had been resolved and it would "move forward on an enforcement path should its investigations identify provable violations".
Massachusetts is the only state that decided not to settle its antitrust claims against the software giant in a landmark case last year.
It argued that the settlement was flawed and is still fighting for tougher sanctions to be imposed.
Microsoft has denied the allegations.
"We are complying fully with all terms of the decree and are working with the appropriate authorities to ensure that this consent decree is implemented," said a company spokesman.
Massachusetts plans to set up a website and a telephone hot line to invite complaints about Microsoft's behaviour.