Officers discovered the inmate was not breathing.
Authorities believed it was an opioid overdose, so officers and the jail medical staff performed life-saving procedures, including administering NARCAN.
Then six other inmates began to show symptoms. They were quickly rolled out of the jail on stretchers and loaded in to ambulances.
Assistant Sheriff Steve McComas says he suspects the drugs came into the jail on Monday.
"It is believed at this point that one of the inmates on that floor recently had been booked into jail and had found a way to smuggle the narcotics into the jail," he said.
Six out of the of the seven inmates have since recovered and are back at the jail. The other is undergoing observation and remains at the hospital.
McComas says he's thankful the officers are trained in administering NARCAN.
"Our nation is in the midst of a huge opioid crisis," he said. "Its's here, it's in Fresno. You can see that if it can get into the jails here in the community, it's a serious matter. The inmate last night was experiencing life-threatening effects of that and luckily, because of the training and the NARCAN available to our officers, we were able to save that inmates life."
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Authorities say this is the first exposure of this magnitude. However, two years ago, an inmate died of a suspected fentanyl exposure.
In order to prevent situations like this from happening again, the sheriff's office bought a narcotics K-9 that they will begin using in the coming weeks.
McComas says the jail also has other measures in place to try to keep all contraband out.
"Obviously, no system is going to be 100 percent effective. We do know that sometimes, contraband does get into our jails but we have a robust system, including body scanners that scan the body upon intake to make sure they are not trying to smuggle things in."