DUJIANGYAN, China — Authorities cordoned off some schools that collapsed in last month's mighty earthquake, keeping out grieving parents and reporters Wednesday in a sign that Beijing was becoming increasingly nervous over accusations of shoddy construction.

Parents whose children were crushed in their classrooms during the May 12 quake vowed to keep pushing the government for compensation, as well as for an explanation of why so many schools fell when other buildings remained standing.

The students' deaths have become a political challenge for the government, which is trying to provide for the 5 million people left homeless amid growing accusations of corruption in school construction.

At Juyuan Middle School, where more than 270 students died, police officers blocked parents from entering the school yard Wednesday. Dozens of people crowded behind police tape, some still waiting for children to be pulled from the rubble.

"There are still bodies in there!" people shouted.

Police refused to say why they sealed off the site: "We're just following orders from above," one officer said, refusing to elaborate.

Until this week, journalists were free to interview parents who held protests and erected homemade memorials at collapsed schools, with many angrily accusing the government of corruption.

But police ordered a half-dozen reporters to stop filming and conducting interviews at the Juyuan school Wednesday. The group was placed on a bus and sent back to the capital of Sichuan province, Chengdu, an hour's drive away.

Parents said the school had been sealed off since Monday, when dozens of families turned their anger on the school principal, who made a rare appearance surrounded by security officials but didn't address the crowd.

Even at a collapsed rural school that had received little media attention, a Chengdu propaganda official stopped two journalists and asked them to leave. "For your safety," Xu Guangjun said.

The quake killed more than 69,000 people, but Lu Guangjin, spokesman for the State Council, or China's Cabinet, said there was no count of how many were schoolchildren.

The government has said about 7,000 classrooms were destroyed, and angry parents and even rescuers have pointed to steel rods in broken concrete slabs that were thinner than a ball point pen.