The State Administration for Industry and Commerce said it opened a case in June after complaints Microsoft improperly failed to publish all documentation for its Windows operating system and Office software.
It said investigators visited Microsoft's China headquarters in Beijing and branches in Shanghai, Guangzhou and Chengdu in southwestern China this week.

"The administration believes the previous investigation cannot eliminate that Microsoft engaged in monopolistic behaviour," the agency said on its website.

Microsoft did not immediately respond to a request for comment.

Foreign technology suppliers face growing pressure from Chinese regulators, who have launched anti-monopoly investigations and announced plans to examine products for security flaws.

China relies heavily on foreign software and communications technology. Communist leaders have expressed frustration about the high cost of foreign know-how and have invested billions of dollars to develop their own phone, encryption, software and other technology industries.

Chinese state media have stepped up criticism of foreign technology companies following disclosures by former National Security Agency contractor Edward Snowden about their possible cooperation with US government surveillance.

In a separate case, state media said last week regulators have concluded Qualcomm Inc, a San Diego, California-based maker of chip sets for mobile phones, has a monopoly.

The reports gave no indication what penalties or demands for changes in its business practices might be demanded.

Regulators said earlier they were investigating whether Qualcomm abused its dominant market position by charging excessive fees for technology.

They announced a similar investigation at the same time of InterDigital Inc, in Wilmington, Delaware, but the results have not been announced.

Also this month, Apple Inc denied allegations reported by state television that the location function on its iPhone might be a national security threat because it could allow access to confidential information.

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