SAN QUENTIN (CBS SF) — The deaths of two more condemned inmates this week on San Quentin’s death row has triggered an investigation by prison officials into the role contraband drugs may have played in the fatalities.

The San Francisco Chronicle reported it had obtained an email sent to the prison’s medical staff, ordering them to a Wednesday meeting to “discuss contraband causing serious fatalities/death.”

(L-R) Joseph A. Perez Jr., Herminio Serna (CDCR)

A state department of corrections officials has confirmed that “officials are investigating how contraband may have been brought into Death Row at the prison.”

Joseph A. Perez Jr. was found unresponsive in his cell at 9:11 p.m. Tuesday. Prison officials said staff members began life-saving measures immediately, but he was pronounced dead 10 minutes later.

Just 24 hours earlier, condemned South Bay killer 53-year-old Herminio Serna was also found unresponsive in his cell and pronounced dead. He was sentenced to death in November 1997 by a Santa Clara County jury that found him guilty of the gang murders of three people.

The Marin County Coroner’s Office will determine the cause of the two deaths. The coroner was already trying to determine if the deaths of condemned inmates Andrew Urdiales and Virendra Govin early Saturday morning were suicides or caused by other means.

(L-R) Andrew Urdiales, Virendra Govin (Illinois Dept. of Corrections, CDCR)

Urdiales and Govin also died within a day of each other in November. State corrections officials had initially said their deaths were unrelated.

California has not executed an inmate condemned to death row since 2006 and has been engaged in a lengthy legal battle over the use of lethal drugs to carry out the sentences.

However, in the last three months, five death row inmates have died.

Jonathan Fajardo, 30, suffered chest and neck wounds from an inmate-made weapon while on the East Block Housing Unit recreation yard complex in October and was pronounced dead a short time later.

The suspect in the attack, authorities said, has been identified as condemned inmate Luis Rodriguez, 34.

Before the recent deaths since reinstating capital punishment in 1978, California has had 78 condemned inmates died from natural causes, 25 committed suicide, 13 have been executed in California, one was executed in Missouri, one was executed in Virginia, and 10 have died from other causes. There are currently 742 offenders on California’s death row.