The CEO of a Montana utility says it's not to blame for a deadly natural-gas explosion in downtown Bozeman five months ago and that the company believes "external forces" caused it. NorthWestern Energy CEO Bob Rowe told Bozeman business owners during a meeting Monday that it's possible not everyone will agree on what caused the March 5 blast that destroyed four buildings and killed one woman. The company is investigating but so are other agencies, attorneys and insurance companies. KBZK-TV reports that Rowe told business owners at the meeting that frosting or abrupt steam may have caused the separation on a 2-inch NorthWestern service line between the main gas line and the meter for the Montana Trails Gallery. However, Rowe said it may be too early to tell for sure and that the utility is still investigating. Ta r a B owma n , t h e 36-year-old director of the gallery, was killed by the blast. The gallery was among the buildings that were leveled. The others were Boodles restaurant, Lily Lu's and Tolstedt Architects. The explosion and ensuing fire also heavily damaged the Rocking R Bar, the Pickle Barrel, the American Legion and the Rocky Mountain Rug Gallery. Many of the downtown business owners who attended Monday's meeting said they're frustrated by how long it has taken to rebuild. Mike Hope, with the Rocking R Bar, said the business lost as much as $4 million because of the explosion. He said they're not financially close to being able to rebuild. In June, people expressed their frustration with the utility by putting a sign near the blast site that read: "Thanks for nothing NorthWestern Energy." Rowe said NorthWestern can't control how insurance companies compensate businesses.