Oklahoma Attorney General Scott Pruitt told ABC News that 30 investigators began checking for fraud in the hours after the tornado hit.
The investigators said they quickly found businesses violating the law, Pruitt said, including a grocery store that was allegedly charging $40 for a case of water.
The state law, which was passed after a tornado hit the same region years earlier, prohibits price increases of more than 10 percent on goods and services like water and hotel rooms for 30 days after a disaster.
"We're looking at everything from work gloves to water to storage units, hotels and car rentals. And long term, we'll be dealing with home construction and repair," Pruitt said.
The Attorney General's office has set up a hotline -- (405) 521-2029 -- for fraud reports.