Chinese authorities said Thursday that four of the six Japanese citizens detained in March for "illegal activities" have been released.

Two others are still being held for accusations of stealing China's classified information and breaking the country's security law and counter-espionage law, officials said.

Hua Chunying, spokesperson for China's foreign ministry, said at a press conference on May 22 that China had investigated six Japanese citizens under suspicion of illegal activities and had notified the Japanese consulate in China in accordance with the China-Japan Consular Agreement.

The six Japanese were either employed by NC Geophysical Survey Co. or He Yuan Hot Spring Development Consultant Co.

The two still under investigation are the heads of the two companies.

Officials said the six had conducted illegal surveying activities in Penglai, East China's Shandong Province and Wuzhi Mountain in Sanya, South China's Hainan Province in March, without any official registration, qualification or permit. They were found with radioactive gamma measuring devices in violation of China's Surveying and Mapping Law and Mineral Resources Law, officials said.

Investigations show that the two companies had conducted illegal surveys in China more than 30 times in the past 10 years.

One of the company's head had illegally surveyed Mount Huangshan in March 2016. The suspect told authorities he was "searching for hot springs."

Investigators discovered classified materials on the suspects' laptops, USB drives and other devices, including more than 80 maps. Two of the suspects are accused of stealing and holding China's classified information, the officials said.