(Bloomberg) — Chipotle Mexican Grill Inc. received a subpoena expanding the scope of a federal criminal probe into a norovirus outbreak at one of its restaurants in California, renewing concerns that its food-safety crisis isn't over.

The subpoena was served on Jan. 28, the Denver-based company said in a statement Tuesday. It requires Chipotle to produce documents dating back to Jan. 1, 2013, deepening an investigation that the company announced in January.

Chiptole's troubles have mounted since the U.S. Centers for Disease Control and Prevention said in November it was investigating an E. coli outbreak linked to the chain. That brought attention to previous cases of Chipotle customers getting sick that hadn't received national attention. Chipotle's stock tumbled, and the crisis raised questions about the chain's protocols for handling fresh ingredients.

CDC announced this week that its probe into Chipotle's E. coli incident was over, giving investors hope that the worst was behind the chain. Still, Chipotle is expected to face higher marketing and food-safety costs as part of its comeback effort. The company is adopting new protocols in its restaurants and supply chain. When it has fully recovered, Chipotle has said it expects to regain its customer traffic and industry-leading margins.

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