"That can put the sources at risk," AEC spokesman Sak Korsaengruang said, adding that the government could also face difficulties finding willing witnesses in the future.

A key witness has been identified in some newspapers, and two suspected gunmen have been arrested on suspicion of planning to murder the witness for giving crucial insight into the Ua Arthorn affordable-housing project in Pathum Thani.

AEC secretary Kaewsan Atibodhi said what had happened would not shake the courage of the AEC in its mission to root out graft.

"We will continue to look into the corruption aspects while police take charge of the masterminds in the alleged assassination plot," he said.

Social Development and Human Security Minister Paiboon Wattanasiritham said he would cooperate with police and the AEC on measures to increase protection for state witnesses.

"These witnesses have come forward for the public good," he said.

The Ua Arthorn scheme is run by the National Housing Authority, which is under Paiboon's ministry.