In a statement Monday, union officials said inadequate staffing at the federal prison has left its correctional officers "in grave danger."
"We can no longer turn a blind eye to the critical situation inside our nation's federal prisons," said Bryan Lowry, president of The Council of Prison Locals of the American Federation of Government Employees, which represents federal correctional officers. "What happened to Jose Rivera sends a clear message that now is the time for change."
Authorities at the prison have refused to answer basic questions about what happened, including how many times Rivera was stabbed, where in the prison he was attacked, the names of the inmates suspected or how many other correctional officers were on duty.
The prison issued a six-sentence statement Friday night saying a staff member had been stabbed and taken to the hospital. It hasn't released any information since.
The high-security prison, which houses more than 1,100 male convicts, has been plagued by staffing shortages.
A scramble to find guards delayed the penitentiary's opening in 2001. In 2006, it was added to the U.S. Bureau of Prisons' accelerated hiring list, putting it first in line to receive correctional officer applications.
It's one of four federal prisons in California offering signing bonuses to fill an "urgent need of correctional officers," according to the Bureau of Prisons Web site.
Some prison officials have argued that the guard shortage hasn't affected safety at Atwater because all of its shifts are filled with employees who work overtime. And Atwater's inmate-to-guard ratio is better than most comparable federal facilities, said Traci Billingsley, a spokeswoman for the Bureau of Prisons.
The average for similar federal prisons is about one officer for every five prisoners, Billingsley said. That compares with a 1-to-4 ratio at Atwater.
Those numbers don't mean much to Rivera's family.
"To me, I don't think it was safe there," said Rivera's mother, Terry Rivera. "There should have been more (correctional officers) with him."
Terry Rivera remembered her son as a loving child who grew into an adult eager to help others.
"He was a good boy," she said.
Rivera was the third of five children. He graduated from Le Grand High School in 2003, enlisting in the Navy shortly after. He served four years, including two tours in Iraq.
He began as a correctional officer about 10 months ago. He lived with his girlfriend in Chowchilla.
Billingsley said she couldn't answer questions about what happened in Atwater the day Ri-vera died, except to say that her department hopes to release soon the names of the inmates thought to be involved.
Rivera's mother said her son wasn't wearing protective gear.
"If they don't have weapons, they should at least have (vests)," she said. "He should have had that."
It's routine that prison guards don't carry guns. The FBI is investigating Rivera's death.
He is the first employee to die in the line of duty at the Atwater prison. His death is the first in 11 years at a federal prison.