Police cordoned off destroyed schools Thursday, one month after China's devastating earthquake, apparently on alert for protests by parents demanding investigations into whether shoddy construction played a role in their children's deaths.

Police barred entry to at least two towns where schools collapsed, despite an assurance by authorities that unfettered coverage would be allowed. A reporter from Singapore's Straits Times newspaper was forced by police to leave the town of Juyuan and return to the provincial capital of Chengdu, about an hour away.

A dozen police and paramilitary troops guarded the gate of Juyuan's destroyed middle school, while a crowd of about 50 gathered outside. It wasn't clear whether any of the parents of dead children were present.

The security measures underscore how public anger over the deaths of so many children is unnerving the authorities.

Some 7,000 classrooms altogether collapsed in the quake, often in areas where no other buildings were badly affected. Parents and some engineers tasked with surveying the wreckage say the collapses appear to point to poor design, a lack of steel reinforcement bars in the concrete and the use of other substandard building materials.

A month after the magnitude-7.9 quake killed nearly 70,000 in central China, Beijing is trying to switch the emphasis off the destruction and onto rebuilding and to tales of heroism in the rescue efforts. Efforts to clamp down on the more open reporting allowed in the initial aftermath of the quake further signal the government's resolve to manage the message.

Across the quake zone, tempers flared among parents. Outside quake-flattened Beichuan, more than 200 parents blocked the valley's sole road, angry that a memorial plaque to dead students had been smashed overnight.

"We're dispirited. Our children have been dead for a month. The memorial plaque is broken and reporters have been chased away," said Wang Ping, whose 16-year-old daughter died when Beichuan Middle School cratered. "I'm 40. All our hopes were in our children. Now they're dead. Our future is dead, too."

The plaque, a shiny polished black stone with gold letters that read "Beichuan May 12 Memorial to Middle School Teachers and Students" had been erected on a hillside near the school in a small ceremony on Monday. Thursday it lay in pieces, and parents were upset that local officials did not seem to care.

In Dujiangyan, across the quake zone, police and troops barred parents from entering the grounds of the ruined Xinjian elementary school.

One family knelt on the sidewalk in front, burning incense and pouring soda into cups as an offering to the dead. They declined to speak to a foreign journalist who slipped past road blocks.

Jing Linzhong, the father of a killed child, said he arrived early in the morning, before security forces sealed the area off, to join other parents in a vigil on the school's playground. Jing said blocking parents from visiting the site could impede the healing process.

"It's unfair," said Jing, seated with three other parents on the playground, surrounded by debris adorned with white funeral wreaths. "Some people are getting psychological counseling, but for us, we find it therapeutic simply to gather at the school and meet with each other. We have a lot in common."

Parents reached in the village of Wufu, where 270 children died in a collapsed primary school, said they were holding off on any commemorations or protests until the release of investigation results promised on or around June 20. The results may open the way for lawsuits or trials against officials and private contractors involved.

"The only thing we are doing now is waiting until the 20th," Li Caojun said in a telephone interview.

Li and another Wufu parent, Ye Yaolin, said they hadn't been threatened or intimidated, although the school site had been closed off by police. Other parents said they had been visited by police and believed their phones were being tapped.

No formal commemorations of the one-month anniversary were being held. In the traditional Chinese mourning cycle, the one-month anniversary of a death is less important than the fifth week, and some parents said they were considering holding ceremonies on June 15 — the 35th day after the quake.

Seeking to aid the recovery effort, a Japanese research body presented materials documenting reconstruction work performed after the 1995 Kobe earthquake to Chinese Ambassador Cui Tiankai, China's official Xinhua News Agency said.

The report quoted Cui as saying the presentation came "at an opportune moment when China has shifted its focus from rescue and relief operations to reconstruction and recovery efforts."