A bus carrying Montana State Prison employees to work struck a deer and overturned Friday morning, killing one person and injuring more than two dozen others, a state Corrections Department spokesman said.

The bus with about 26 people onboard came to rest on its right side around 5:30 a.m. in the median of Interstate 90 near Warm Springs, about 50 miles southwest of Helena.

Nurses driving to work at the prison stopped to help, Anaconda Police Detective Bill Sather said.

Montana Highway Patrol Trooper Rodger Kenney said a front tire of the bus hit the deer and it appeared the driver struggled to control the bus for a time before it overturned. He doesn't expect any charges to be filed.

"It was a simple deer crash that turned chaotic," Kenney told KUFM radio in Missoula.

Several ambulances and two helicopters took at least 21 people to hospitals across western Montana with head wounds, broken bones and cuts, Sather and hospital officials said. At least two victims were in serious condition, said Michael Walsh, chief executive of Powell County Medical Hospital in Deer Lodge.

By Friday afternoon, 16 prison employees remained hospitalized and nine had been treated and released, said Bob Anez, a spokesman for the Montana Department of Corrections.

The victim who was killed, Sonja Ryan, 56, had worked for the state since 1981, Anez said. Ryan worked in the prison's records department.

The bus driver had a broken leg, said Tom McGree, co-owner of Tucker Transportation, which carries the workers for the state.

Staff at the prison stayed beyond their regular shifts to ensure all posts were covered, and the facility was operating normally and securely, Warden Mike Mahoney said. The prison has about 1,380 inmates and 630 employees.

The Montana Highway Patrol was investigating.