Len Iwanski Associated Press

Three railroad tank cars containing poisonous chlorine gas derailed and ruptured near this Western Montana town on Thursday, forcing the evacuation of hundreds of people and sending at more than 100 to hospitals.

A cloud of chlorine gas hovered over the area throughout the day, and emergency officials anxiously watched to see whether the plume would dissipate or move.

Bill Reed, chief of the Missoula Rural Fire Department, said late Thursday afternoon the plume was stable and did not threaten other nearby communities, including Missoula, about 30 miles away and at 45,000 one of Montana’s largest cities.

Of 91 people taken to four area hospitals, 11 were admitted and two were listed in critical condition Thursday evening. The other 80 were treated and released. Most had lung irritation and difficulty in breathing.

Gov. Marc Racicot declared an emergency in Missoula and Mineral counties, allowing the National Guard and other state agencies to assist local governments if necessary.

The derailment involved a 71-car Burlington Northern-Santa Fe train pulled by five locomotives. It was operated by a Montana Rail Link crew on Rail Link track and making its way east from Pasco, Wash., with an eventual destination of Houston.

Eighteen cars derailed about 4:30 a.m., including five tank cars that tipped onto their sides, Rail Link spokeswoman Lynda Frost said. Four of them contained chlorine gas.

Only one was believed immediately to have ruptured. However, when a team of specialists wearing protective gear were able to reach the scene, they found three of the cars were breached and were releasing chlorine. The entire 170,000 pounds of chlorine in one tank car was gone. The other two vented more slowly.

Interstate 90, which runs next to the derailment site, was closed. Traffic was rerouted on an 80-mile detour.

All 400 people in Alberton were evacuated, as were rural residents for miles around.

Frost said Montana Rail Link was paying for food and lodging for the evacuated people and had summoned emergency response teams from several private companies. She said cause of the derailment was not immediately known.