Chinese authorities have formally arrested two Japanese men, who were detained earlier this year while conducting geological assessments for hot spring developments, a Japanese government source said Friday.

The two were arrested last month, the source said, though the charges were not known.

The Global Times, a nationalistic tabloid affiliated with the Chinese Communist Party, reported in July that the two are suspected of having violated the country’s national security and anti-espionage laws.

The two men and four other Japanese nationals in China were detained in March while they assisting in geological assessments at sites in Shandong and Hainan provinces after receiving orders from two Chinese hot spring developers.

The four were later released by Chinese authorities and have already returned to Japan.

Chinese authorities have retrieved a large amount of classified information, including nearly 80 copies of maps, from the pair’s computers and other electronic devices, the Global Times said.

In a separate case, a Chinese court held its first trial session in August for a Japanese man indicted this year on an undisclosed charge, according to the source.

The first hearing for Hideji Suzuki, head of a Japan-China youth exchange association, is believed to have taken place at a court in Beijing. It was closed to the public, the source said.

Suzuki is known to have developed friendly ties with China, but was taken into custody on suspicion of endangering national security while staying in Beijing in July last year to make arrangements for a symposium.

Such allegations are often applied in cases that China regards as involving espionage or related activities.

China has been tightening its watch over foreign nationals and organizations since President Xi Jinping came to power in 2013.

A number of foreign nationals have been arrested on similar charges, a figure that has only grown since a counterespionage law came into force in 2014 and a new national security law took effect the following year.

Since 2015, at least 12 Japanese nationals were held on such allegations or for unspecified reasons.

Currently, eight remain in custody and Japanese officials have confirmed that Chinese closed-court hearings have already begun for five of them, including Suzuki.