Three Chinese dairy companies have publicly apologized for their involvement in a toxic milk scandal that has killed at least four children, according to news reports. Inner Mongolia Yili Industrial Group, Mengniu Dairy and Bright Dairy Group were found to have produced milk contaminated with melamine, a compound that can be used to cheat in tests to detect watered-down dairy products. "I feel I have let everybody down. I have done so much, yet still done wrong," the Monday edition of Beijing News quoted the Mengniu marketing chief, Zhao Yuanhua, as saying on state television. Zhao and executives from Mengniu and Bright Dairy also promised consumers that their prices would not rise despite higher costs of quality controls, according to reports.