Six inmates at San Quentin State Prison have been diagnosed with Legionnaires’ disease as of Sunday, officials said.

An inmate was diagnosed Wednesday with the severe type of pneumonia, California Department of Corrections and Rehabilitation officials said. By Sunday, there were six confirmed cases with five inmates being treated at an “outside hospital,” officials said in a news release.

An additional 51 inmates are under observation for respiratory illness but have so far not been diagnosed with Legionnaires’ disease, according to the release.

Officials with the Marin County Public Health Department, California Correctional Health Care Services and CDCR continue to investigate the source of the disease, officials said. Visiting time for prisoner’s family members has also been halted, they added.