Chipotle Mexican Grill Inc., revealed in a filing that it has been served with a grand jury subpoena as part of a criminal investigation tied to a norovirus outbreak in August at its Simi Valley restaurant in California that sickened at least 234 people. The investigation is being conducted by both the U.S. Food and Drug Administration and the U.S. Attorney's Office in California, the company said in a filing with the Securities and Exchange Commission Wednesday, according to the Washington Post.

The subpoena, received in December, requires the company to produce a "broad range of documents," about the Simi Valley restaurant, the source of August's outbreak.

Chipotle spokesman Chris Arnold said Chipotle intends to fully cooperate with the investigation, but declined to go into detail citing the company's policy about pending litigation. "As a matter of policy, we don't discuss pending legal action, but we will cooperate fully with this investigation," he said, according to ABC News.

The FDA released a statement discussing its role in the investigation but didn't go into specifics due to the ongoing criminal complaint. The U.S. Attorney's Office declined to comment for similar reasons. 

"When foodborne illness outbreaks occur, the FDA works closely with other federal and state agencies and other health officials to identify the source, ensure that companies are removing affected foods from the marketplace, and communicate with the public," the FDA said. "No system is foolproof, but the FDA works to prevent as many foodborne illness outbreaks as possible."

Chipotle has been fallen on hard times ever since concerns over the safety of its food products began, reported CNBC. On Wednesday the company announced that it expects comparable restaurant sales to fall by about 15 percent in the fourth quarter - a 4 percent increase from what it had established before. This metric dropped as much as much as 37 percent in December alone.

In an attempt to rehabilitate its image, the company has apologized to its customers in full-page ads in dozens of newspapers around the country, and has vowed to step up food safety standards at its restaurants.

Despite that, some experts believe Chipotle will take years to recover if at all. "I'm not sure, quite frankly, they'll ever have the halo they did prior to the outbreak," said Bob Derrington, senior restaurant analyst at Telsey Advisory Group.

"It could take a decade before they're fully back," said Gene Grabowski, a partner at K Global and the head of its crisis communications group.