The second incident in the space of a few days at Minnesota's maximum security prison injured five more staff members.
The Minnesota Department of Corrections confirmed to Bring Me The News that five staff at MCF–Oak Park Heights, near Stillwater, were taken to the a hospital on Sunday evening.
It follows an incident involved a single offender.
It came just two days after a melee involving an unknown number of offenders led to five other prison staffers getting injured.
In a statement, Minnesota DOC spokeswoman Sarah Fitzgerald said: "I cannot provide any further details about the incident because it is being actively investigated.
"We have exceptional people working in an extremely tough environment who put themselves at risk every day to keep the public safe. They do very difficult and honorable work."
MCF-Oak Park Heights opened in 1982, and according to the DOC website it has a capacity of up to 473 inmates.
It's the state's only maximum-security prison, holding the highest custody level in the DOC system.
The majority of those held there are classified as maximum custody or "extreme risks to the public."