The incident happened around 9:17 p.m. on a ride called the 'Vortex,' which is along the lower (or original) midway section behind the Expo Center on the west side of the fairgrounds near gates 4 and 5.
Wake County Sheriff Donnie Harrison said an initial investigation showed the ride had stopped and people were getting off when it suddenly started up again.
He said two people suffered serious injuries and three others had minor injuries. Some of the people injured may have been family members.
All the victims were between the ages of 14 and 39 years old. Authorities said they requested their names not be released.
Two of the injured remained hospitalized in critical condition, according to The Associated Press.
AP reports Tom Chambers, the chief of the ride inspection unit at the state Labor Department, said that the ride's operator was not injured. At the time of the malfunction, the operator was at the controls and the attendant was helping people off, he said.
An eyewitness told ABC11 that he had just gotten off the fair ride when it started up again. He said he then saw two young victims fall.
The witness said a boy was knocked unconscious, but was awake when paramedics arrived. He said a young girl appeared to have a head injury, and the ride operator fell about 20 to 30 feet onto his side.
The man described the scene as people "domino-ing out of the ride."
"I heard three or four bangs, I mean it sounded like baseballs hitting an aluminum floor," witness Jonathan Stanley said. "I just ran over and I saw the bodies, I mean, I was right at the fence. There was one guy in particular; he was all the way back against the back of the ride. It looked as if he had flown out and maybe hit it and then maybe came down face first."
"They were falling like rain drops," Max Bryn said. " A person fell and landed head first on the ground and was just completely knocked unconscious."
Witnesses also said one ride attendant was visibly upset.
"They were hitting the metal board right in front of us where the ride was on, non-stop," Bryn said.
"The ride attendant was crying, just torn up ... was on his knees ... all shaken up," Stanley said.
"I'm sure he thought it had something to do with him and it may or may not, but I'm sure that in that moment he felt guilty," Stone Gully said.
N.C. State Fair officials said they do not train ride operators, the ride companies do, and they just monitor that the operators are paying attention.
"It appears to be an accident and things like this happen," Harrison said shortly after the incident. "We hate it because it is supposed to be a fun time for families."
However during a news conference Friday morning, Harrison said they are treating their investigation as criminal -- as they do every investigation.
He said the investigation is ongoing and will "take time" because they have a lot of witnesses to interview.
Harrison has urged anyone with video of the accident or the aftermath to contact the Wake County Sheriff's Office.
Meanwhile, North Carolina Governor Pat McCrory has offered his support to the victims and the investigation.
"Our sincere thoughts and prayers are with those injured in last night's State Fair accident and their families," McCrory said in a statement. "The state stands ready to assist in any way we can to help those impacted and ensure the safety of our citizens and fairgoers."
Fair officials said they will be looking at diagnostics and all the safety systems of the ride.
They also revealed a complaint about the 'Vortex' ride was filed Monday night about a bad switch on the system that locks passengers into the seat. They said a repair was made and the ride was later reopened.
The Department of Labor said the rides are inspected three times a day.
The ride was manufactured by Technical Park International of Italy. The ride is owned by Family Attractions out of Georgia. This is the first time it had been at the N.C. State Fair.
Fair spokesman Brian Long told ABC11 he was not sure who actually operates the ride.
There are two rides named 'Vortex' on the fairgrounds, but fair officials said they are different.
It is not the first time an accident involving a ride has happened at the N.C. State Fair.
The most recent one occurred in 2009 when a worker fell while trying to break down the 'Flying Bobs' ride.
Years earlier in 2002, a worker was killed when he slipped and was hit by the 'Bonzai Ride.'
And in 1998, three riders were hurt in an accident on the 'Zyklon' roller coaster. Three cars collided when a wheel bearing on one of the cars seized up. The riders suffered minor injuries in that incident.
In the meantime, the 'Vortex' will remain closed. The fair is operating on normal schedule.
Fair officials said they hope Thursday's "isolated thing" will not impact people enjoying the last weekend of the fair.
"I am confident my grandchildren can ride the rides ... and they will," State Agriculture Commissioner Steve Troxler said.