(CNN) — Austin police are investigating a sudden surge in suspected opioid overdoses in the Texas capital, with dozens of reported cases in just two days this week.

While the exact source of the overdoses remains unclear, “It is apparent that there is a deadly batch of illicit narcotics in our community,” Assistant Police Chief Eric Fitzgerald said Tuesday.

From Monday morning through midday Tuesday, authorities responded to 51 opioid overdoses, according to Austin-Travis County Emergency Medical Services. Typically, the county sees only two or three overdoses a day, EMS Assistant Chief Steve White said.

Emergency responders were unable to revive four of the victims. And the local medical examiner’s office is investigating whether four other deaths may have also been caused by overdoses.

Toxicology results are pending, but authorities suspect the drug at the root of the overdoses is fentanyl, Austin Police Lt. Pat Eastlick said.

The source of the suspected “bad batch” has not been found. Two people were detained in connection with the investigation, Eastlick said, and one was charged with felony possession of a firearm. But no charges have been filed related to the overdoses.

“Anyone found responsible for distributing the suspected fentanyl faces potential charges of murder or manufacture or delivery of a controlled substance causing death (or) serious bodily injury,” Eastlick said.

Nationwide, the drug overdose crisis has killed more than 100,000 Americans a year.

In recent years, many overdose deaths have been caused by opioids – drugs formulated to replicate the pain-reducing properties of opium.

Some opioids are legal, such as the prescription painkillers morphine, oxycodone and hydrocodone – though they can be dangerous when abused. Other opioids are illegal, such as heroin and illicitly made fentanyl.

Illegal stimulants such as methamphetamine and cocaine are playing an increasing role in the overdose crisis. Experts have called this the?“fourth wave”?of the opioid epidemic.

While the first wave of opioid overdose deaths was driven by prescription opioids, the second wave by heroin, and the third wave by synthetic opioids, this “fourth wave” is marked by extremely potent synthetic opioids like fentanyl and concurrent stimulant use.

About 112,024 people died from a drug overdose in the 12-month period ending in May 2023, according to estimates from the US Centers for Disease Control and Prevention’s National Center for Health Statistics.

That’s an increase of more than 2,700 drug overdose deaths compared to the previous year. About 109,261 people died from drug overdoses in the 12-month period ending in May 2022.

While authorities investigate who and what is behind the suspected overdoses in Austin, Travis County Judge Andy Brown encouraged everyone in the community to be prepared if they see someone who may be in the throes of an overdose.

“Everyone should carry Narcan and know how to respond to an overdose,” he said.

CNN’s Mira Cheng and Amanda Musa contributed to this report.

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