Amid an ongoing E. coli outbreak investigation at Chipotle Mexican Grill, the Food and Drug Administration (FDA) on Tuesday announced that it is joining the effort to investigate what may be a second wave of illnesses linked to the chain restaurant. The new illnesses are caused by the same type of E. coli found in the previous cases—Shiga toxin-producing Escherichia coli O26 (STEC 026)—but with a different, rare genetic variant.

To investigate this second wave, the FDA has combined forces with state and local authorities, plus the Centers for Disease Control and Prevention (CDC), which on Monday first announced an investigation into five cases of the variant E. coli infections. Those cases included one in Kansas, one in North Dakota, and three in Oklahoma. The sickened people from Kansas and North Dakota reportedly ate at the same Chipotle restaurant in Kansas before falling ill. The three sickened in Oklahoma were separate cases, but all three reportedly ate at the same Chipotle, the FDA reported.

The new cases, if confirmed, would bring the new E. coli outbreak numbers to 58 sickened and 12 states affected. The other states linked are California (3 cases), Illinois (1), Maryland (1), Minnesota (2), New York (1), Ohio (3), Oregon (13), Pennsylvania (2), and Washington (27). All of the cases involve some form of the STEC 026 bacteria.

Usually, the germ causes severe diarrhea, often bloody, and abdominal cramps two to eight days after a person eats contaminated food. But STEC 026 can also cause more severe illness, which can lead to kidney failure. There have been no reports of death in the outbreak.

Based on the data so far, the FDA and CDC suspect that a common ingredient or menu item is the source of all recent STEC 026 cases. The agencies, along with local and state officials, are using whole genome sequencing to track the germ. But to date, the source is unknown.

Amid the initial STEC 026 outbreak, a Boston Chipotle restaurant was linked to a separate outbreak involving norovirus, the most common cause of gastrointestinal illness in the country. In that outbreak from earlier this month, 153 people were sickened, including 141 students from Boston College. Local health officials reported a number of health safety violations at that restaurant, including meat held at the improper temperature and a sick employee.

In response to the outbreaks, Chipotle has laid out new guidelines to guard against further illnesses, including expanded food testing and updated safety procedures. In November, during the initial STEC 026 outbreak, the restaurant chain closed 43 restaurants in Washington and Oregon. With the approval of local health officials, those restaurants have since reopened.

Still, a Reuters/Ipsos poll on Tuesday found that a quarter of people aware of the outbreaks said they were eating at the restaurant chain less often than usual. Also, the burrito-maker’s stock has plummeted in response to the outbreaks. On Tuesday, its stock closed below $500 for the first time since May 2014.