But Idaho Power spokeswoman Lynette Berriochoa says the damage might have occurred anytime in the last several months and could have been accidental.
She says the line failed immediately on Wednesday when extra electricity went through after a secondary line was taken out of service.
The last time the line carried additional power was April 30.
The outage affecting 17,000 customers included the town of Hailey, which on Wednesday canceled a planned celebration for the end of June marking the return of Bowe Bergdahl after five years of captivity in Afghanistan.
The company is reporting the vandalism to authorities.