Suspected drug overdoses at a Northern California prison over the weekend left one inmate dead and sent 11 others for treatment, authorities said Monday, wrote latimes.

An inmate was found unresponsive in his cell Sunday at Mule Creek State Prison in Ione, but despite lifesaving efforts he was pronounced dead later that evening, according to a statement from the state Department of Corrections and Rehabilitation.

Another inmate was treated at the prison and 10 others were taken to hospitals, although eight have since returned to the lockup.

Authorities didn't say what drug they suspect caused the overdoses. But the cells where the overdoses occurred have been sealed and are being cleaned by a hazardous materials team.

Authorities are trying to determine how the drugs were smuggled into the prison. Prison officials are limiting the movements of inmates, and medical and prison staff members are conducting wellness checks throughout the facility every 30 minutes, according to the statement.

In 2014, prison officials began a program to combat pervasive drug smuggling that gave California an overdose fatality rate three times the national average. The number of overdose deaths peaked at 24 in 2013 and has since fallen to 19 per year among the state's prison population of about 130,000.

As of last year, the state had spent about $15 million to thwart prison drug smuggling through increased surveillance using cameras, urine tests, drug-sniffing dogs and airport-style scanners.

Even so, smuggled drugs continue to be a problem, whether passed by corrupt guards, tossed over fences or concealed in the mouths of visitors.

A public report last year found mixed results from the efforts. It said about 8% of inmates tested positive for drugs in random testing over a six-month period beginning in July 2014. Opiates, methamphetamines and marijuana were the most commonly detected.

The study also said guards found illegal drugs about 5,000 times from May 2015 to February 2016, and some drugs were even found at San Quentin's death row.

The seized drugs included methamphetamine in a bar of soap, drugs hidden in housing unit shower drains, and marijuana in a refrigerator and a trash bin near a visitors' center.