DENVER - Staffing at the Supermax prison has gotten so low that job hazards have increased for correctional officers watching over the nation's worst terrorists, an arbitrator has ruled. The arbitrator stopped short of ordering the Bureau of Prisons to hire more staff, but union officials representing Supermax officers said the ruling would bolster their argument to Congress for more prison funding. "If the most maximum security federal penitentiary is indeed understaffed, what is happening across the entire Bureau of Prisons as far as staffing levels?" asked state Rep. Buffie McFadyen, who testified for the union at an arbitration hearing in May. Her district includes Supermax and 11 other state and federal prisons. The American Federation of Government Employees had said assaults and threats increased after staffing at the Supermax was reduced last year. Two inmates were killed in two separate incidents. The slayings were the first since the prison 90 miles southwest of Denver opened in 1994. It houses the nation's most dangerous inmates, including al-Qaida member Zacarias Moussaoui, Unabomber Theodore Kaczynski and Oklahoma City bombing conspirator Terry Nichols. Housing units at Supermax require three officers per shift, but entire units were sometimes left unstaffed, and cells were not being searched regularly, the arbitrator found. Arbitrator Joseph Lazar ordered the bureau to reduce inherent hazards at Supermax to the lowest possible levels, as called for by a collective bargaining agreement. "The agency in fact did not keep its bargain to lower hazards to the lowest possible level," Lazar wrote in a ruling made available Friday by union officials. Supermax officials were not immediately available for comment.