CHICAGO (AP) — Illinois regulators say they are investigating apparent violations of a rule intended to prevent marijuana shops statewide from stockpiling weed from a single cultivator while also reminding retailers that they must keep enough products on hand for medical patients.

The Illinois Department of Financial and Professional Regulation sent a letter to dispensaries Friday saying they were probing whether stores were illegally sourcing more than 40% of their product from one grower.

Some dispensaries struggling with the supply deficit say they were worried that companies that own both dispensaries and cultivation sites could be attempting to limit product available to competitors.