The investigators found a "very complicated" trojan, or backdoor, in the hacked network, with characteristics like those previously identified as U.S. intelligence spyware.
The hackers aimed to steal earthquake monitoring data to serve military needs, said Du Zhenhua, senior engineer at China's National Computer Virus Emergency Response Center.
Earthquake monitoring data is a high-value target for U.S. military because it contains detailed geographic data.
"This attack was a planned military operation carried out by U.S. intelligence agencies," Du told Global Times.
The center provides freely accessible information on earthquakes, but the detailed sensor data is not accessible, which may help foreign tests and weapon calibrations.
The hackers could trigger incorrect earthquake alerts to spread chaos among civilians or mess with the sensor data during an earthquake and cause bigger loss from the disaster.
The U.S. has built a "real-world matrix" to steal secret from everyone, allies included, as files previously disclosed by the U.S. intelligence agencies showed.
"In fact, the U.S. has long been conducting all-round network spying on China's critical IT infrastructure," Du said. "In this immoral way, they want to contain China's development and maintain its dominance."
The investigators said they will disclose more details about the U.S. surveillance system in the near future.