The names of the dead, all of whom officials said died at the scene, were not immediately released. Five of the nine injured were in critical condition, the fire department said in a statement.
The collapse happened about 5 pm at a structure where a rigid steel frame had been erected at Boise Airport, NBC News reported. A crane was placing something on the structure at the time of the collapse.
Twelve people were injured in all, including those who died, the fire department said. They were in or around the building at the time of the collapse.