Shoddy construction and poor planning contributed to the toll from China’s devastating earthquake, engineers studying the disaster for the government said Thursday in Beichuan, China, as a lake still swelling behind a wall of debris threatened communities downstream.

Premier Wen Jiabao flew by helicopter to oversee efforts to drain Tangjiashan lake, which was formed when a landslide set off by the May 12 quake blocked a river.

“Now is a critical moment for the Tangjiashan quake lake, and the most important thing is to ensure there are no casualties,” the state-run Xinhua News Agency quoted Wen as saying.

More than 10,000 people in low-lying areas about 30 miles downstream were evacuated by Thursday afternoon, adding to a total of more than 250,000 who had been forced to flee to higher ground, Xinhua said.

Engineers and building experts sent to the disaster zone to study damage are raising questions about poor construction, bad urban planning and lack of enforcement of building codes. The problems were especially glaring at some schools and in rural areas and small towns, they said.