A mailroom supervisor at a California prison has died after coming into contact with mail "saturated in an unknown substance", the US Federal Bureau of Prisons said.

Marc Fischer - a Coast Guard veteran who had worked as a prison employee for 23 years - began to feel "unwell" after he was exposed to the substance at Atwater Prison in California. He died after he was rushed to a hospital, a spokesperson for the agency said.

A second worker was also exposed and hospitalised, but has been released, the spokesperson added.

"On Friday, August 9, USP Atwater and the Federal Bureau of Prisons, lost one of our own," the Bureau of Prisons said.

"Tragically, despite the swift response from his co-workers and local emergency medical services, Supervisory Correctional Systems Specialist Marc Fischer was pronounced deceased upon arriving at the local hospital."

Authorities have launched an investigation into Fischer's death. Law enforcement is trying to determine whether the mail may have been saturated in fentanyl, according to the Associated Press.

Visitations at the California facility have been suspended until further notice, according to the prison website.

Atwater prison is a high-security facility for men in Merced County, California, 130 miles (209 km) from San Francisco. Around 1,200 people are incarcerated there.

Fischer spent his career at the California prison, working as a correctional officer for three years before being promoted to Supervisory Correctional Systems Specialist in 2009, the Bureau of Prisons said.

Fischer leaves behind a wife, son, daughter and "countless grieving friends and colleagues", the agency said.