Chinese police have questioned the wife of Ai Weiwei, the artist and activist has said.

Four officers arrived at their home in Beijing and asked Lu Qing to accompany them to the police station for questioning, Ai said. He was out at the time but has since spoken to her by telephone.

Lu was released after three hours of questioning, Ai told Reuters.

He told the Guardian that police had also questioned two of his assistants recently, adding: "It seems they are doing broader research or something – I have no idea."

He said: "We are not nervous. We have not by any means violated the law. But we are curious as to what will come out because we never know."

Ai and his supporters believe his 81-day detention in spring and the £1.5m tax bill that officials handed him this month were payback for his social and political activism, while Chinese officials say his case is unrelated to human rights. State media have said he was detained for economic crimes.

Although tax authorities have held him personally responsible, the alleged tax evasion relates to Fake, the company which handled his affairs but was registered in Lu's name.

Lu was first questioned in April, days after Ai was detained at Beijing airport.

But Ai added: "I think the tax would not be the problem because we have followed the instructions and paid the bond [the first required payment] and they seemed quite satisfied. For them that was winning a victory.

"That's why I feel it's not dangerous for her. If there was anything they were going to do it would come to me directly."

Several of Ai's friends and associates were also detained for months this spring.

The artist added: "I'm worried, but nothing can help because when my arrest and release happened, it never had an explanation. When you know your worry will not help then you have to give it up. This is not something where you can find a solution.

"It seems there are two different logics co-existing in this world. They must have a strong reason but we will never have it and they will not communicate."

A police officer answering the phone at Nangao police station said he did not know who Lu was and was unaware of any detention. Calls to Beijing police rang unanswered.