Six weeks after China's devastating earthquake in May, a group of volunteer social workers arrived in the rubble of Fuxin Number Two Primary School and started meeting parents of children killed when the school collapsed in the tremor.

At first they seemed like any of the other 1.3m Chinese citizens who rushed to the quake zone in the immediate aftermath in an unprecedented outpouring of civic involvement.

But some parents quickly decided something was wrong with this latest group of “volunteers”.

“We asked to see their identification, but they wouldn't show it to us and although they were quite nice they kept telling us not to make trouble,” said one parent, who asked not to be named for fear of reprisals. He said the five volunteers repeatedly urged parents to stop demanding an investigation into why the school was so poorly built and why it collapsed in the May 12 quake when most of the buildings around it remained standing.

Many other parents were also suspicious of the opinionated social workers.

“They were definitely sent by the government to keep an eye on us and identify the troublemakers,” said one parent, who also asked not to be named.

In the immediate aftermath of the quake the parents of the 131 students killed in Fuxin Primary, in Sichuan's Mianzhu City, south-west China, were among the most vocal in demanding an investigation.

The photograph of a government official kneeling before the angry, protesting parents from Fuxin Primary was published in Chinese media and quickly became a potent symbol of growing outrage over the 7,000 classrooms that collapsed.

Soon after the photo was published, security services broke up protests and the government banned reporting on the issue.

The crackdown contrasted sharply with the period following the tremor when citizens from all over the country travelled to the epicentre to offer their services.

They were spurred on by 24-hour television coverage as state-run media reported on a natural disaster for the first time in China's history.

But while propaganda officials directed the media to hail the selfless spirit of the volunteers – with its echoes of the idealistic early years of Chinese communism – the government quietly closed off the opportunities for volunteer participation.

Partly for logistics reasons and partly out of an ingrained reflex to control all facets of public life the government quickly required all volunteers to register with the authorities and operate through an approved organisation. Soon it became clear that some types of volunteers were unwelcome.

One of these was a human rights campaigner, Huang Qi, who was arrested one month after the quake, for “possession of state secrets” after he made more than 10 trips to the quake zone carrying food, water and medicine to survivors.

During those trips he advised grieving parents, including those from Fuxin Primary, on how to pursue a legal campaign against the government and wrote about their grievances on www.64tianwang.com, his website.

His wife, mother and associates allege his arrest was punishment for focusing on such a sensitive subject when the government was trying to suppress widespread indignation at the shoddy construction of collapsed schools.

Mr Huang is no stranger to persecution, having spent five years in prison for campaigning on behalf of parents whose children were killed in the nationwide crackdown that followed the protests centred on Tiananmen Square, Beijing in 1989.