Ai Weiwei's wife said Tuesday she was summoned by police and interrogated for three hours as a "criminal suspect", becoming the latest person close to the Chinese artist to be pulled in for questioning.

Lu Qing's interrogation comes after police also questioned two of Ai's assistants. She said police had asked her about Fake Cultural Development, the company at the centre of a tax row that Ai founded but which she legally owns.

Ai, whose widespread activism has made him a thorn in the side of China's Communist authorities, disappeared into secret police custody for 81 days in April this year.

After his release he was charged with tax evasion related to dealings by Fake Cultural Development, and this month he was handed a bill for 15 million yuan ($2.4 million).

The 54-year-old has denied the charges, calling them "politically motivated", and said he intends to appeal.

Within weeks of receiving the bill, he was able to pay an 8.45 million yuan guarantee needed to challenge the charge thanks to donations from fans.

Lu -- who was last questioned by police when Ai was in detention -- told AFP after she was released that police had summoned her as a "criminal suspect" and told her not to leave Beijing for the time being, though she was not sure why.

"What they asked me was very similar to what they have asked me before -- who family members are, what my job is," she said.

She added they had also asked her about Fake, with questions revolving around design costs and money surrounding construction projects.

"But why is Beijing police asking me about this? It should be the tax bureau," she said.

Beijing's municipal police department would not comment on her case when contacted by AFP.

Ai said this month that police were also investigating possible charges of pornography against him, centred around pictures taken of the artist and four women -- all naked -- although Lu said police had not asked her about that.

The artist's problems with authorities started when he began investigating the collapse of schools in the 2008 Sichuan earthquake and a 2010 fire at a Shanghai high-rise that killed dozens.

But until his detention he had been left relatively unscathed thanks to his family background. His father is the late Ai Qing, a famous poet who was disgraced and later rehabilitated by the Communist regime.

Speaking to AFP earlier, Ai said he was worried about Tuesday's turn of events.

"If I have done anything wrong, it has nothing to do with her (Lu). They should directly come to me, not to her," he said.

"We paid the bond, they seemed very happy with it, but maybe they're trying to threaten her, or to tell her authorities are there. I have no idea."