Singh said he had received complaints that wheat meant for the Public Distribution System (PDS) provided by the Centre was sold back to the state at the minimum support price by shopkeepers.
Singh urged Thomas to get samples of wheat from Madhya Pradesh and find out if it had come from the PDS.
The Congress leader in a two-page letter to Thomas yesterday also wanted to know who was responsible for the delay in supply of gunny bags for wheat storage to Madhya Pradesh.
Seeking a probe into the manner in which businessmen purchased wheat from the farmers, Singh alleged that the state government had purchased only 60 per cent of the wheat produced.