McDonald's Corp. and Mattel Inc. said they have been investigating an outbreak of violence last week at a Chinese factory that supplies them with toys but haven't found evidence that the incident was a result of poor treatment of workers. The investigation into the factory comes after a report by China Labor Watch...said more than 1,000 of the plant's workers had rioted to protest poor wages and working conditions. The factory, owned by Merton Co.,...makes toys and promotional items that are sold by McDonald's and Mattel as well as other foreign companies, including Walt Disney Co... Gary Rosen, a spokesman for McDonald's in China, denied the incident was caused by Merton's labor practices and said McDonald's believes the incident is an isolated one. The investigation determined that a disgruntled worker who had been recently laid off returned to the plant and, together with a group of his friends, committed several acts of vandalism, Mr. Rosen said. McDonald's will continue to monitor conditions at its factories, he said... Mattel spokeswoman Lisa Marie Bongiovanni said Mattel and other companies had sent auditors to the Dongguan factory to look into the incident. She said those investigators have "not been able to substantiate the claims" made by China Labor Watch. A spokeswoman for Disney said the company is also investigating, but didn't elaborate. All three companies require their suppliers to follow certain labor standards and submit to regular inspections of factory conditions. "This is a reputable factory," said Mr. Rosen, of McDonald's. "We're there because they adhere to a code of conduct."