Chinese police say the company of the detained artist Ai Weiwei has evaded "a huge amount" of taxes and intentionally destroyed accounting documents, state news agency Xinhua reported.

Officials had earlier said police were investigating Ai in connection with suspected economic crimes, but his family and friends believe he has been targeted because of his social and political activism.

According to the brief Xinhua article, police said the information came from their initial investigation into the company, called Beijing Fake Cultural Development Ltd.

Ai's wife, Lu Qing, has said the company is actually hers, although it handles his affairs. She did not respond to calls.

The company's accountant, Hu Mingfen, also went missing in April, as did Ai's driver and cousin Zhang Jinsong, his friend Wen Tao and colleague Liu Zhenggang.

Former workers at the studio have described Hu as diligent, with one telling the Guardian: "She is very prudent. Lu Qing used to say: 'Even if it's just one cent missing, Hu has to count it to be clear.'"

Last year, Ai told the Guardian that the state might take action against him and that security officials had been to his bank.

Officers also confirmed to Xinhua that Ai was under residential surveillance – the first official comment on his status since he was stopped by officials at Beijing airport on 3 April. They said that gave him the right to meet people who lived with him, presumably in reference to his brief meeting with his wife on Sunday – the first time he had been seen since he disappeared.

Lu said he appeared in good physical health but mentally conflicted and tense.

Although residential surveillance is normally carried out at a person's home, the law is not clear on what constitutes a residence. The measure allows police to hold people for around six months, while detention allows them to hold people for only one before reporting to prosecutors.

A foreign ministry employee who answered its out-of-hours number hung up twice. The ministry has previously said Ai's detention is nothing to do with human rights.