On 3 April, shortly after Ai Weiwei's detention, a concerned contact tweeted the artist's friend Wen Tao to ask if he was ok.

"So far so good," replied Wen, who had been spending much of his time at the studio. An hour later three men dragged him into a black car in the Caochangdi art district of north Beijing and drove away.

Wen, 38, has been missing for seven weeks now, his detention just a few hours shorter than that of the world-renowned artist. Yet his case has barely been reported. It has sparked no protests overseas; no politicians have stood up to condemn it.

"Ai is grabbing a lot of attention but people like Wen Tao are not as famous," said Wen's girlfriend Shi Jing. "[They] can do whatever they want [to him] and therefore the danger for him is bigger. I just hope that more people will pay attention to this case. It's related to Ai's. As long as Ai's case is not clear, he [Wen] won't be released."

Wen is one of four missing people who friends believe are being held simply because of their connections to the artist. Chinese authorities have now allowed Ai to see his wife, albeit briefly, and have apparently acknowledged that he is being held under residential surveillance.

Yet his four associates – Wen, driver Zhang Jinsong, accountant Hu Mingfen and designer Liu Zhenggang – have vanished entirely. Family members have repeatedly tried to register them as missing, only to be turned away by police. Supporters believe the authorities hope to extract testimony from the four that can be used against Ai.

On Friday police alleged Fake Design, the company handling Ai's affairs, had evaded "huge amounts" of tax and destroyed accounting documents. The artist's family say he has been targeted for his social and political activism.

While Ai is a big, burly figure Wen – known as Tommy to many – is small and slight. A former sports journalist, Wen, who wore distinctive orange sweatshirts and drove a matching car, moved to covering news but lost his job at a state newspaper after reporting protests by artists about the forced demolition of their property.

Although he travelled with Ai, took part in his projects and spent a lot of time at the studio, friends insist he was not an employee.

"He [Wen] was very connected to it, but he felt he was participating as an independent journalist," said Alison Klayman, the filmmaker working on a documentary about Ai. "He felt he was documenting things, keeping a record. I think he was really trying to figure out a new model of journalism."

Another friend, Ah Ding, recalls Wen buying tents and foods for survivors of the Yushu earthquake. "He is a simple, kind and warm-hearted person. Sometimes he would even cry when talking about people's suffering," he said.

Despite his own social activism, supporters are convinced Wen's detention is linked to Ai's because of its timing. And they believe the cases of the others are even more straightforward.

"Apart from Wen Tao, none of these [missing people] cares about politics," says a former worker at the studio. Hu "doesn't read news and has no sense of politics", added Shi, who has also volunteered at Ai's studio.

But as the accountant for Fake Design, the 55-year-old is key to any investigation into the company. Similarly Liu, 49, is an architect who spent little time at the studio but may have held a position at Fake. His wife declined to comment on his case, but friends say he was taken away on 9 April.

Zhang is thought to have gone missing in the early hours of 11 April, shortly after police went to his apartment. In the previous week he had repeatedly asked police for information on Ai, who is his cousin as well as his employer.

The 43-year-old, known to friends as Xiao Pang, grew up in the north but moved to Beijing with his mother - Ai's aunt - to help care for Ai's dying father.

"He is a very honest person. Zhang's mum is worrying about him every day and crying," said Gao Ying, Ai's mother.

"He [Zhang] is one of those people who get stuff done. He is so nice, one of those guys who has your back," said Klayman.

Police in Beijing did not respond to faxed queries about the four missing people, while a foreign ministry press officer referred queries to the relevant local authorities.

The foreign ministry has said that Ai's detention is "nothing to do with human rights".

• This article was amended on 23 May 2011. Shi Jing was misspelled as Shi Qing. This has been corrected.