Two things still haunt Wang Bin a year after an earthquake decimated his village: the death of his teenage son and a purchase he made more than a decade earlier.

He bought the cartful of bricks and two tons of cement in 1995 from a contractor who said they were extra material from a school construction project. The price was low, and Wang's neighbors had already taken advantage of the deal.

It was only after the magnitude-7.9 earthquake devastated Sichuan province last May 12 — razing towns, collapsing classrooms and taking almost 70,000 lives — that it hit the hollow-eyed, 45-year-old laborer.

"That material was actually meant for the school," Wang, a former construction worker, said recently at his temporary home near the ruins of Beichuan Middle School.

He and other parents say the contractor sold them the material for personal profit. They contend that steel reinforcement rods, bricks and pillars they found in the rubble show that he skimped or used shoddy replacements. Hundreds of students, including Wang's son, died when the school collapsed, the destruction a stark contrast to the intact buildings close by.

"He used his position to sell those things on the side," Wang said. "At the time, no one suspected there was a problem."

One year after China's biggest earthquake in three decades, reconstruction is moving full speed ahead. But one question remains: Why did almost 7,000 classrooms collapse — killing thousands of students — when many buildings around them remained standing?

The death of so many children has touched a nerve nationwide, raising questions about corruption and mismanagement that have flourished amid China's breakneck economic growth — and whether the country's one-party dictatorship would ever accept blame.

He Xiaogang, an engineering expert from Tsinghua University who was on a team of government investigators that visited quake sites, said the sheer power of the earthquake is to blame for the number of flattened schools.

"We went to tens of thousands of schools, and almost all of them were up to national standards," He said.

But an assessment by American engineering experts who visited the quake site last August with Chinese counterparts said many of the school buildings were reportedly unreinforced structures — brick or block walls without steel — that had been outlawed after another massive earthquake near Beijing in 1976.

"As would be expected, this type of building did not perform well in the earthquake, and there were many catastrophic collapses," the experts from the Earthquake Engineering Research Institute and the Geo-Engineering Earthquake Reconnaissance Association — both based in California — said in their report.

Authorities have clamped down on information about the school-related deaths and have never released a death toll from the schools. Parents or activists who have sought information have been intimidated or detained.

"I'm filled with a sense of hopelessness," said a father in the town of Wufu, where the Fuxin No. 2 Primary School caved in, killing 127 students. The man, who would only give his surname Liu, said a group of parents had filed a lawsuit in Sichuan but had not heard back from the courts. Another group traveled to Beijing to petition the central government and have since been under tight surveillance.

The efforts have drawn the sympathy of activists, many who have been questioned or harassed by authorities for helping the parents.

Ai Weiwei, an avant garde artist and high-profile critic of Beijing's policies, has been trying to compile a list of dead students. So far, he has confirmed almost 5,000 names and estimates that the toll could reach 8,000. At least 20 of his helpers have been detained by local authorities, he said.

"In order for China to move forward, we must not shield our eyes, but rather talk about the problem and have transparency," Ai said. "If such responsibility cannot even be taken by the government, a society like that is very dangerous."

Many parents say they have been warned against protesting or talking to foreign reporters. An Amnesty International report chronicles instances in which dozens of parents were questioned or detained by police while seeking answers from courts and local officials. It also said lawyers who took on such cases were pressured into dropping them.

Wang said he and other parents met with lawyers from Beijing last year and showed them construction plans that detailed the amounts of building material intended for the middle school. They also signed statements saying they had bought cement and steel reinforcements from Lu Wanchun, the contractor.

The lawyers promised to call back, Wang said. They never did.

"I've talked to so many people. I've talked so much that I don't want to talk anymore. Nothing I say will have an effect on anything," Wang said.

He perked up only to describe his son, Huaiqiang, who at 19 was taller than his father and studied hard to get into a good university. "I never found my son," Wang said. "He never even managed to escape the building."

Minutes into the interview, Wang asks an Associated Press team to leave, fearful that local authorities will harass him for speaking out.

Reached by telephone, Lu said selling materials from construction projects was very common, and that the cement he sold to parents was going to expire soon. He said he also gave away some steel beams. But, Lu insisted, what he did had no effect on the quality of the construction.

"My son studied in Beichuan Middle School for six years and there's no way I would build a school of low quality," said Lu, who lives with his family in a nearby town and still works in construction. "The school was a big project at the time and the whole county paid great attention. The school principal oversaw the building, which went through several inspections."

Lu said he did not remember how much material he sold and dismissed Wang and other parents as "people who are mentally unbalanced because their children died in the quake."

When asked about the school damage, the Beichuan county government reiterated Beijing's line that the quake was the main reason for the collapse.

Throughout the quake corridor, once-shuttered towns buzz with reconstruction work. Trucks and tractors are parked next to piles of gravel and bricks as builders work in the heat and dust. Ubiquitous red-and-white government banners urge courage and speedy recovery.

In Dujiangyan, a town southwest of Beichuan famous for its ancient irrigation system, Zhou Lekang quietly mourns his 16-year-old son, who is buried on the banks of the river he grew up swimming in. A small mound of dirt rising from the grass is the only sign of the boy's grave.

Zhou, a high-cheekboned construction laborer with a shy smile, keeps Jingbo's photos and handwritten essays in a torn manila envelope and shows them with pride to visitors.

"Our whole mind and soul were with him, our only child," Zhou, 43, said as he recalled how he dug for hours through the rubble of Juyuan Middle School, where nearly 300 children were killed. "We think of him everyday."

He said residents paid a construction fee of 400 yuan per square meter for the school although the cost of the materials was much lower.

Juyuan was overcrowded, and teachers banned students from running or jumping, Zhou said. "With just a little shaking, dust would fall off the walls."

Back doors in classrooms were sealed, so students sitting in the last few rows were trapped during the quake, he said. Later, when parents — many of them construction workers — sifted through the debris, they found building materials of questionable quality.

"The cement was of a low grade. The steel reinforcements were not only thin, but also appeared to have been bought from scrap markets," Zhou said.

Parents in Duijangyan have been among the most vocal in their quest for justice. Right after the quake, police pulled grieving families away from a courthouse where they knelt in an attempt to submit a lawsuit.

"They have controlled our freedom of speech," Zhou said. "Parents would welcome it if government give a grand commemoration on one-year anniversary. But they dare not do it. They have no guts."