Police in Austin, Texas, said they responded to an “outbreak” of over 50 opioid-related overdose calls between Monday and Tuesday morning that’s been linked to as many as eight deaths.

Authorities said they started receiving an increase in opioid calls around 9 a.m. Monday. It was concentrated in the downtown area but spread throughout the city, said Austin-Travis County Emergency Medical Services Assistant Chief Steve White.

EMS usually receives an average of 2 to 3 calls a day, White said at a news conference Tuesday afternoon. During the surge, authorities responded to about 51 calls, he said.

The overdose patients range in age from their 20s to 50s, according to White.

“We have not experienced overdoses of this volume since 2015 when K2 struck our community,” he said, referring to synthetic marijuana that resulted in mass overdose.

Eric Fitzgerald, the Assistant Chief for the Austin Police Department, said it is “apparent that there is a deadly batch of illicit narcotics in our community.”

Paramedics distributed over 267 doses of Narcan, an opioid overdose reversal drug. Heidi Abraham, the deputy medical director for Austin, said at a news conference Monday that Narcan was the reason “some of these people are alive today.”

White said among the patients police responded to, four died from suspected overdoses. The Travis County medical examiner said he is involved in eight deaths that “are suspicious for a drug overdose.”

Police are still investigating the overdoses and said they detained two persons of interest. One of them was charged with felony possession of a firearm.

Further investigation is needed for additional charges, Fitzgerald said.