United Natural Foods Inc. (UNFI), a major food distributor for Whole Foods, Cub Foods, and co-op grocers nationwide, was hit by a cyberattack last week that forced parts of its system offline and delayed shipments to stores across the country.

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UNFI discovered “unauthorized activity” on its network on June 5 and quickly shut down some systems. Since then, the company has been working to restore operations and bring ordering and receiving platforms back online.

In the meantime, store shelves have gone empty. Whole Foods locations across the U.S. reported shortages in frozen foods, dairy, and other essentials. “The attack disrupted our supplier, UNFI, which is causing some temporary product shortages,” Whole Foods said in an internal staff message.

Some smaller grocers and food co-ops said they’ve had to turn to other vendors like Sysco, Amazon, and Walmart to try to keep products in stock.

UNFI is working with law enforcement and cybersecurity experts to investigate the attack. CEO Sandy Douglas told analysts that the company had implemented workarounds and was slowly recovering key business parts.

“We are making progress every day,” Douglas said. “Some systems are further along than others.”

UNFI expects to resume core operations by Sunday, June 15, but full order volumes may take longer to return. The company has also ended its supply agreement with Key Food, one of its major retail customers.

The cyberattack comes at a tough time for UNFI. The company had already been dealing with cost-cutting efforts and recently announced layoffs. After news of the incident, UNFI’s stock price dropped by as much as 17%, losing nearly $300 million in market value.