Boise, Idaho - The hunt is on for a white supremacist who was sprung from custody in a shoot-out at an Idaho hospital that left three prison staff wounded, police said Thursday.

Skylar Meade was being escorted out of a medical facility after receiving treatment when gunfire erupted, seemingly from inside the hospital's emergency department.

Boise Police Department said an attacker – named as Nicholas Umphenour – shot two prison officers, leaving one critically wounded, before fleeing with Meade in a car.

Responding officers initially mistook a prison staffer for a target and opened fire, wounding him.

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"This brazen, violent, and apparently coordinated attack on Idaho Department of Corrections personnel to facilitate an escape of a dangerous inmate was carried out right in front of the Emergency Department," said Boise Police Chief Ron Winegar.

Winegar said officers had rushed into the emergency department, believing Umphenour to be inside, locking down the hospital as they searched.

"We caution anyone who may have contact with the suspects involved in this violent attack and escape to call 911 immediately and not confront them personally," he said.

"They are armed, dangerous, and have shown they are willing to use extreme violence in furtherance of their criminal activity."

Meade, a member of the Aryan Knights white supremacist gang who has distinctive facial tattoos, was jailed in 2016 after attacking a law enforcement officer.

His prison sentence was not due to end until 2036.

Winegar said that all three wounded prison staff are expected to make a full recovery.