SAN QUENTIN (CBS SF) — Condemned killer Orlando Romero, sentenced to death along with his brother Christopher Self in 1996, died Sunday at a hospital outside of San Quentin while being treated for a COVID-19 infection, according to state prison officials.

Romero is the 11th death row inmate to die while being treated for COVID-19 since the virus outbreak swept through San Quentin beginning in June. The last execution at San Quentin was in 2006.

READ MORE: Report Reveals San Jose State University's $4 Billion Economic Impact On California

Also Read: Released Inmates Describe Ordeal Inside San Quentin During Coronavirus Outbreak

Prison officials said Romero’s death appears to be from complications related to COVID-19, but a coroner will determine his exact cause of death.

READ MORE: UPDATE: Coroner Identifies Man Killed in Juneteenth Mass Shooting at Oakland's Lake Merritt

A second general population inmate also died of virus over the weekend, raising the death toll from the San Quentin outbreak to a prison system high 21. As of Sunday, there were 147 active COVID-19 cases among San Quentin’s inmate population and 1,965 convicts have recovered from the exposure to the illness.

Romero and Self went on a three-month-long crime spree in Riverside County’s Mead Valley that involved the deaths of at least three people.

The 47-year-old Romero was sentenced to death in Riverside County on Aug. 28, 1996, for first-degree murder and second-degree robbery while armed with a firearm. He was also sentenced to three life-with-parole sentences: one for attempted first degree murder armed with a firearm, another for attempted first-degree murder and other numerous offenses, and the third for kidnap/robbery while armed with a firearm and several robbery offenses.

Two of the murder victims, Joey Mans, 26, and Timothy Kipp Jones, 24, both of Riverside were shot to death execution-style Oct. 12, 1992, near Lake Mathews. Jose Aragon, 22, of Redlands, was killed when shots were fired from three weapons Nov. 25, 1992, in San Timoteo Canyon near Beaumont.

MORE NEWS: Police Need Help Finding Sacramento Teen Who May Be In Oakland

There are currently 715 people on California’s death row.