The Department of Justice is eyeing IBM for possible antitrust violations in the mainframe computer market.

Officials at a computer industry trade group comprised of IBM rivals and partners told Reuters and other outlets that the group, which has filed past complaints about IBM's mainframe dominance, was contacted by DOJ investigators in recent days.

The Computer and Communications Industry Association contends that IBM unfairly withholds documentation and other data that competitors need to make their systems interoperable with IBM mainframes.

CCIA members include Microsoft, Google, Oracle, Red Hat, and AMD.

Reports indicate the DOJ's investigation is in the preliminary stages and may not lead to a full blown, formal probe of the nation's largest tech company. Just last week, a judge dismissed an antitrust complaint against IBM filed in federal court in New York by mainframe maker T3, also a member of CCIA.

The judge found that IBM's refusal to license mainframe technology to third parties does not constitute anticompetitive behavior. IBM officials said the ruling supports their contention that the comapny does not use monopolistic practices to promote its mainframe business.

"We continue to believe there is no merit to T3's claims, and that IBM is fully entitled to enforce our intellectual property rights and protect the investments that we have made in our technologies," a spokesman told Reuters. The spokesman added that IBM would "cooperate" with the Justice Department.

Mainframe computers once dominated corporate data centers, but in recent years have given way to more economical, modular servers. Still, mainframes remain a lucrative business for IBM as customers in high-end niches like financial services and scientific research remain willing to pay top dollar for the most powerful systems.

On Wednesday, President Obama awarded IBM a national Medal of Technology and Innovation for its work developing the Blue Gene supercomputer, one of the world's fastest systems.

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