HUNTINGTON, W.Va. (AP) — Officials in a West Virginia city are warning people about an especially dangerous batch of heroin after authorities responded to 26 overdoses in within a four-hour span.

The rash of overdoses came Monday in the city of Huntington, which sits in Cabell County along the Ohio River in the western part of the state. Gordon Merry, the county’s EMS director, said at a news conference Tuesday that the heroin the users had taken was laced with a strong substance, but authorities aren’t sure what it is.

Many of the overdoses were in an area surrounding one apartment complex in the city, he said, leading officials to believe the cases were connected. He said the amount of calls that were received overwhelmed responders.

“Just to give you an idea, when the first few came in, three ambulances were already out dealing with overdoses,” Merry said.

For a half-hour span, there were no ambulances available in the county to send, Cabell County EMS assistant supervisor David McClure added.

Merry said eight of the victims were revived Monday using the opioid-overdose-reversing drug naloxone and others by a manual resuscitator called a bag valve mask to stimulate breathing. One victim was given three doses of naloxone.

Cabell County responded to 39 overdose calls in all of August 2015.

There have been more than 440 overdoses in Huntington from all types of drugs this year through mid-July.

“As a public health problem, this is an epidemic of monumental proportions,” Dr. Michael Kilkenny, director of the Cabell-Huntington Health Department, said. “We really must stop the demand side of the equation. We must attack the issue of addiction.”