Cyberattacks that stole massive amounts of information from military contractors, energy companies and other key industries in the U.S. and elsewhere have been traced back to a Chinese military unit, according to a U.S. security firm.

The Associated Press reports that China’s Foreign Ministry dismissed the report as “groundless,” and the Defense Ministry denies any involvement in hacking attacks.

The Virginia-based Mandiant Corp.’s report contains some of the most extensive and detailed accusations to date linking China’s military to a wave of cyber espionage against foreign companies and government agencies, AP reports.

Mandiant says it traced the hacking to an office building run by “Unit 61398” of the People’s Liberation Army, and that the unit “has systematically stolen hundreds of terabytes of data from at least 141 organizations. The report adds that the unit has been in operation since at least 2006.