The city government in Mianyang, Sichuan , is tightening surveillance on activists and petitioners ahead of the upcoming one-year anniversary of the May 12 earthquake, a source of sorrow for parents who lost their children. A document released by the city's Department of Justice ordered a blanket screening by the end of tomorrow for key suspects who could undermine social stability. Official surveillance was also ordered on them in accordance with information in a database. 'Those who have been deemed to undermine social stability or are identified as trouble-making petitioners should be put under watch and constrained within their communities in their hometowns by one or many enforcement officers,' said the directive, dated on Friday. The directive is the latest in a string of official measures by authorities to clamp down on dissent to make the occasion a showpiece for official reconstruction efforts. However, as the anniversary edges closer, authorities are increasingly uneasy. Activists and aggrieved parents are expected to push for justice over the thousands of children who died because of shoddily built school buildings. An internal briefing believed to be from the provincial Department of Construction imposed a ban on evaluations of buildings that collapsed in the earthquake, which could dash parents' hopes of getting justice. About 88,000 people were killed or went missing in the magnitude 8 quake, but authorities have so far refused to hear parents' grievances over the construction quality. Activists from some non-governmental organisations have been harassed by police in Mianyang and many other areas in Sichuan for collecting evidence to support allegations of substandard construction materials or data on the number of students killed in the quake. Wang Xiaodong, a Chengdu -based volunteer who has been working with parents trying to prove that some of the buildings at Beichai Middle School in Mianyang were built with substandard materials, said earlier that Mianyang police had come to his neighbourhood to find him. Mr Wang, who did not answer his mobile phone last night, claimed in his blog that police had come to his parents' home, questioning his father about his whereabouts. Mr Wang - who uncovered a crucial construction chart of the Beichuan Middle School, which could shed light on the poorly built school buildings - said in the latest posting, dated Saturday, that he would rather kill himself than fall into the hands of the police. Ai Weiwei, an outspoken mainland artist, said in his blog that volunteers collecting data on the number of student deaths in the quake were being constantly harassed by police. His blog entries documenting the number of students killed in the quake have been subjected to official censorship.