An airport hangar under construction on an airfield in Idaho has collapsed killing three people.
Nine others were injured in the incident which happened at around 5pm on Wednesday on the grounds of Boise Airport, officials said.
Five of the injured are in a critical condition and receiving care at local hospitals.
Boise Fire Department, Boise Police, Gowen Field Fire Department, and Ada County Paramedics responded to the incident.
“Tonight, there was a catastrophic collapse of a metal structure at a construction site,” said Aaron Hummel, Boise Fire Department operations chief.
“First responders found a hectic scene and worked to secure and rescue victims.
“The Boise Airport, city of Boise, and all first responders extend their deepest sympathies to those impacted.”
Everyone at the site has been accounted for, he said, and the cause of the “large-scale collapse” is being investigated.
“I don’t know what caused it, but I can tell you it was a pretty global collapse,” he said.
Some of the victims were on a hoist or elevated platform at the time the structure fell, and that required some specialised rescue efforts, he said.
He confirmed that a crane also collapsed during the incident.
Boise Airport operations were unaffected, and no city-owned buildings were impacted by the incident.
The names of those who died will be released by Ada County Coroner’s Office pending notification of next of kin, officials said.
Firefighters remain at the scene, the site is secure and there is no threat to the public, officials said.
Terra Furman was driving on Interstate 84 at about 5.30pm when she spotted at least 20 police cars, ambulances and fire engines near the entry to the airport.
They were around what she described as a crane folded in half and a building collapsed into the shape of an “M”.
“The walls were still up at a point and the middle collapsed in on either side,” she said.
James Quintana was driving to the airport when he saw emergency vehicles rushing past him.
He said he immediately thought it was a plane crash. He then saw the collapsed hangar and paramedics tending to victims.
“I’m retired law enforcement and when there is that much commotion, that many emergency personnel and vehicles, there is something huge that has taken place. It was a scary sight,” he said.
The hangar collapsed next to Jackson Jet Centre, which offers private aeroplane charters and maintenance.
Boise city records show the contractor Big D Builders had obtained permits to build a 39,000 sq ft jet hangar for Jackson Jet Centre.
The $6.2m (£4.9m) project was to include the construction of a concrete foundation and a metal building.
Jessica Flynn, CEO of Jackson Jet Centre, said in a statement their “hearts go out to everyone affected by this horrific event”.
She said dozens of people were working on site.
“We do not know exactly what caused the hangar collapse,” she said.
“Our focus now is on supporting our team and partners during this difficult time.”