LOS ANGELES (CBSLA.com) — Chipotle says it has been served with a federal grand jury subpoena in connection with a criminal investigation involving an isolated norovirus outbreak at their restaurant location in Simi Valley.

On Wednesday, Chipotle Mexican Grill, Inc., announced the U.S. Attorney’s Office is requiring the restaurant chain to “produce a broad range of documents” related to the incident that occurred in Aug. 2015.

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“We intend to fully cooperate in the investigation,” Chipotle said in a written statement. “It is not possible at this time to determine whether we will incur, or to reasonably estimate the amount of, any fines, penalties or further liabilities in connection with the investigation pursuant to which the subpoena was issued.”

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Chipotle is also facing additional problems due to E. coli outbreaks that were linked to restaurants in several states, including California.

The criminal investigation is being conducted by the U.S. District Court for the Central District of California in conjunction with the U.S. Food and Drug Administration’s Office of Criminal Investigations.

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The investigation is restricted to the Simi Valley norovirus incident, a Chipotle spokesperson told Reuters.