DENVER, CO - The Colorado federal prison dubbed the nation's most secure has dangerously low staffing levels, making it more difficult to guard notorious criminals such as Sept. 11 conspirator Zacarias Moussaoui and the Unabomber, a union representing its officers said Thursday.

Staffing was reduced at Supermax in February 2005 and shortly after, two inmates were killed in two separate incidents, according to the American Federation of Government Employees.

The slayings were the first since the prison 90 miles southwest of Denver opened in 1994 with a staff of 220 that has now dwindled to 185, the union said. The prison houses about 400 of the nation's most violent and disruptive inmates, including Eric Rudolph, Ted Kaczynski and Terry Nichols on "bombers' row."

"I feel that we do protect society from convicted terrorists. Once our resources are actually diminished then it makes it a little harder to do that," correctional officer and local union president Jeff Johnson said following an arbitration hearing to discuss the union's complaints.

Krista Rear, a spokeswoman for Supermax, couldn't immediately confirm the union's staffing figures but said that overall staffing at federal prisons has decreased. She said the cuts were made to make operations more efficient and effective but that safety hasn't been compromised.

"At no time is the public in danger as a result of having fewer staff here. All our security measures and procedures are able to be fully followed with the numbers of staff that we have," Rear said.

Unlike state prisons, Rear said that all employees at federal prisons like Supermax _ from nurses to secretaries _ are trained to respond to emergencies within the prison and know how to use a shotgun, an M-16 rifle, and a 9mm handgun.

According to the union, decreased staffing has increased tensions between officers and prisoners, leading threats to officers to double from 55 between March 2004 and 2005 to 110 between March 2005 and 2006.

Johnson said prisoners are sometimes being denied privileges like recreation breaks and telephone calls because there aren't enough officers and that causes more tension.

Rear said the prison didn't keep track of the number of threats against officers, only assaults. She said she wasn't aware of anyone being denied privileges.

Moussaoui entered the triangular, two-story prison last week. Prisoners are kept isolated in soundproofed, 7-by-12 foot cells. Each cell has a long, narrow window looking out at other prison walls or the small concrete recreation yard.