The narcotic suspected of killing an inmate and sickening 12 other prisoners at Mule Creek State Prison last month was fentanyl, tests conducted by the state Attorney General's office have found.
The apparent overdoses took place at the state lockup in Ione (Amador County) on April 21 and 22. Ten of the inmates who became ill were treated at outside hospitals.
Since the overdoses, state prison and Amador County officials repeatedly declined to release the name of the drug involved in the case, citing an ongoing investigation.
But on Monday that changed.
"The Department of Justice, Bureau of Forensic Services, tested suspected narcotics that were recovered during the investigation into recent overdoses at Mule Creek State Prison," Krissi Khokhobashvili, a spokeswoman for the California Department of Corrections and Rehabilitation (CDCR) said in an email. "The substances tested positive for fentanyl."
The drug is believed to have killed Beau Hooper, a 45-year-old inmate who was serving a sentence of life with the possibility of parole for first-degree murder in San Diego County.
Hooper started his sentence in 2002. It was his second time in a state lockup. Starting in 1996, he served a four-year sentence for second-degree burglary and vehicle theft.
Hooper was found unresponsive in his cell and later pronounced dead, according to state prison officials.
His defense attorney in his murder case, Inge Brauer, did not respond to a request for comment.
The overdoses prompted an investigation by CDCR and the Amador County District Attorney's office. That probe is being monitored by the Office of the Inspector General (OIG), which oversees the state prison system, according to OIG spokesman Shaun Spillane.
Drug overdoses are the fifth-leading cause of death inside the prison system, according to state corrections officials and the second-leading cause of non-chronic illnesses.
Drug use in state prisons is a major problem, according to inmate advocates.
"There's an incredible amount of drugs that enter the prison system," said Donald Specter, executive director of the Prison Law Office.
In recent years, the prison system has worked to reduce the amount of drugs smuggled into state prisons by testing inmates, using mechanical scanners and dogs to sniff out drugs.
CDCR officials note that the agency offers substance abuse treatment and all of the state's prisons offer other self-help groups focused on recovery.
But Specter says those efforts are not paying off.
"There's no effective interdiction program, and there's no effective treatment program to help prisoners get away from opioids or other addictive drugs," Specter said.
In 2016, 29 inmates died from drug overdoses. That year marked the highest rate of drug overdose deaths since 2006, according to CDCR spokeswoman Vicky Waters. Of those cases, 27 were attributed to opioids or methamphetamine.
Between September 2015 and July 2016, seven state inmates died from apparent fentanyl overdoses inside the prison system, according to reports compiled by the OIG.
KQED's Peter Jon Shuler contributed to this report.