Andrew Urdiales, 54, and Virendra Govin, 51, were found unresponsive at the San Quentin State Prison on Friday and Sunday, respectively.
Urdiales — who was pronounced dead early Saturday and had been on death row since Oct. 12, was sentenced to death on Oct. 5 by a jury in Orange County for killing five women in California.
He previously faced the death penalty for three murders in Illinois. Former Illinois Gov. George Ryan commuted his sentence to life in prison in 2011 after capital punishment was outlawed in that state.
Urdiales was discharged from the U.S. military in 1991 but committed his California murders while he was stationed at various Marine Corps facilities in Southern California.
Govin, found unresponsive and alone in his cell in a separate death row housing unit on Sunday, was sentenced in December 2004 for committing four Los Angeles County murders, then setting the victims’ family home on fire.
Govin arrived on death row in January 2005. His brother, Pravin, has been on death row since September 2005.
There is no indication that the deaths of Govin and Urdiales are related, officials said on Monday.
California has not executed anyone since 2006 and inmates are far more likely to die from suicide.
There have been 25 since 1978 when California reinstated capital punishment — or old age, of which there have been 79 deaths.