Chipotle Mexican Grill Inc. executives said Tuesday that last week’s norovirus incident in a Virginia restaurant was caused by a worker ignoring protocol and coming in sick. And they laid out plans to combat recurrences of the incident as well as to try again to win back customers scared away by this and past foodborne-illness problems.

The Denver-based company on Wednesday said it received a follow-up federal grand jury subpoena last week in connection with the Virginia outbreak. (More below.)

The announcement came during the Denver-based restaurant chain’s second-quarter earnings call, in which executives reported a second straight quarter in which they boosted revenue, same-store sales and diluted earnings per share by at least 8 percent from the year before.

But Chipotle (NYSE: CMG) CFO Jack Hartung cautioned that sales have fallen off by about 5.5 percent in the week since news broke July 18 that as many as 135 people were sickened after eating at a restaurant in Sterling, Virginia, near Washington, D.C.

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