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Authorities are warning about a counterfeit pill containing a lethal amount of opioids believed to have caused four overdose deaths in San Diego County this week.

The fatalities occurred within 24 hours across four locations in Valley Center, Santee, Poway and Lakeside, San Diego County Undersheriff Mike Barnett said in Thursday afternoon news conference obtained by KTLA sister station KSWB in San Diego.

The deaths have been tied to imitation oxycodone pills known as M-30s or blues. These tablets are known to contain fentanyl or carfentanil, both of which can be fatal even in the smallest dose.

"One pill could easily contain enough fentanyl or carfentanil to kill someone," Barnett said. "It only take 2 micrograms of fentanyl to be a lethal dose."

The pills are small, light blue and round. They're scored with an "M" inside a square on one side, and the number 30 over a line dividing it in half on the other.

They're meant to mimic prescription oxycodone.

"That pill looks remarkably like prescription oxycodone hydrochloride," Barnett said. "So it's being sold as oxycodone. Whether or not the consumer knows that it contains fentanyl or carfentanil, we don't know at this point."

Detectives are still trying to determine how many calls for service they've had this week involving the pills, including non-fatal incidents. They're also investigating who's supplying them.

Similar pills have been smuggled into the U.S. Southwest from Mexico by the Sinaloa cartel, giving them the nickname "Mexican oxy," the Associated Press reported in February.

That same month, the Drug Enforcement Administration seized more than 20,000 of the pills in New York City. The agency said they sell on the street for up to $30 each.