Government ministers were quick to blame poor engineering standards as the main reason, but the project's developers did not confirm the government's assessment. Media reports and government statements have described the key structure involved as a saddle dam, which controlled the flow to a second, much larger clay core and rockfill dam with a bigger catchment area and reservoir.

Whatever occurred, it triggered flash floods hitting villages downstream from the Xe Pian River and resulting in death and destruction in the Sanamxay district of Attapeu province in southern Laos. The body of an infant was retrieved on Sunday.

Xe Pian is one of the tributaries of the giant Mekong river. The project, which is expected to produce 410 MW of electricity, had been under construction since 2013 and was 90% complete at the time of the collapse, said government sources in Vientiane, the national capital. The project is being led by a private developer on a build-operate-transfer basis.

An image of the hydroelectric dam project, posted on the development company's website, where there was apparent saddle dam failure or partial failure. The exact cause and origin of the deadly flooding has not been determined.

There are 120 dams on the Mekong and its tributaries in China, Thailand, Cambodia and Laos.

Engineering assessments in a document archive say that the project's main components consist of a large reservoir impounded by a dam on the Xe-Namnoy River, a long underground waterway to develop a high head and an open-air switchyard and powerhouse. There is also a straight tailrace channel that connects the powerhouse to the Xe Kong River.

The main structure, the Xe-Namnoy Dam, is 1,600 meters long and 73.7 m high, with a catchment of 522 sq kilometer. Project documents also indicate that the flow to the main dam will be controlled by another, smaller "concrete gravity and clay core rockfill dam," the Xe-Pian Dam. It is 48 m high and 1,307 m long. The two are connected by a transfer conduit.

A third saddle dam is also part of the project, but the trouble is believed to have started at the Xe-Pian Dam.

Parts of the project were still under construction by SK Construction of South Korea, one of the development partners. Firms from from Thailand and Laos also were involved in work at the site.

Prime Minister Thongloun Sisoulith has appointed an ad hoc committee to investigate the cause. The committee has been asked to find out whether the collapse was triggered by heavy rainfall or the engineering standards of the dam.

"The Ad Hoc Committee will assess the losses and find out the true causes," he said.

The local Vientiane Times said the ad hoc committee will seek participation from the partners of the development company behind the dam project as part of its investigation.

In the "the past day, teams have found dead bodies, which put the death toll at 31 while the number of missing is at 130," Ounla Xayasith, the deputy governor of Attapeu, told reporters Sunday.

Rescue and Relief Challenges

Rescuers and relief givers are working against great odds because of the heavy mud and lack of proper equipment and infrastructure in the remote area. 

The floods took down the power supply system in five of the affected villages, according to Vithanga Phommachanh, director of the energy and mine department of the province. The government is trying to provide emergency power supply in shelters and hospitals, he said.

The tragedy has raised serious questions about the construction of large dams in poor countries with poor support infrastructure and lack of ability to deal with disasters. The Xe-Pian Xe-Namnoy project is one of the 10 major dam projects in Laos. The disaster is expected to weaken the  government’s efforts to complete work on two mainstream dams at Xayaburi and Don Sahong, observers said.

The project company, PNPC, is a joint venture among SK Engineering and Construction, Korea Western Power, Ratchaburi Electricity Generating Holding and Lao Holding State Enterprise.