RALEIGH, N.C. (AP) -- Five people were injured on a carnival ride known for its wild twirls and flips at the North Carolina State Fair, and officials were trying to determine Friday exactly what caused the accident. Two people remained hospitalized in critical condition after the "Vortex" started up again as riders were getting off late Thursday evening, officials said. Three other people sustained less serious injuries. Among the possible causes for the accident that investigators will be reviewing is a safety switch that malfunctioned on the ride Monday, according to Tom Chambers, the chief of the ride inspection unit at the state Labor Department. The ride was temporarily idled as workers replaced the switch, Chambers said. It reopened Monday night after being tested. Two of the injured remained hospitalized in critical condition, Chambers told a news conference. He would not discuss their injuries or give their names, although he 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. State agricultural commissioner Steve Troxler, whose agency runs the fair, said he remains confident in the safety of the rides. He stressed that the accident was "an isolated incident." "Safety is something we take very seriously," he said. "We are all shaken by this." Kelly Deis, a spokeswoman for WakeMed Hospital in Raleigh, said earlier that two people were treated and released from the facility. She did not give out further information. The injured riders ranged in age from 14 to 39, Wake County Sheriff Donnie Harrison told a news conference. The "Vortex" spins, twirls and flips passengers upside down. Long said the ride would undergo inspection by the state Department of Labor, and that the sheriff's office would also conduct its own investigation and look for witnesses. Powers Great American Midway is a carnival company that manages the rides. The owner, Les Powers, told The Associated Press on Friday he didn't know what caused the accident. "Nobody wants this to happen, and we're trying to find out why this did happen. Until I find out any information, I can't even give you a clue," Powers said. His contract calls for him to provide 93 rides to the fair. His company has 54 rides at the fair, and it contracted with other companies to provide the rest. The Vortex is owned by a Georgia-based carnival owner, not Powers, he said. "Oh my God, we feel absolutely horrible for the families," he said. "I'm not used to this. This doesn't happen. I don't know how to react. I tried to sleep last night, but I couldn't. I just feel so horrible for them, about the whole thing. I wish I could turn the clock back. " The N.C. Department of Labor is investigating the accident, spokesman Neal O'Briant said. "We're looking at the mechanical equipment and the operation of it to try to determine what happened," he said. Investigators will also interview witnesses. The department inspected the rides before the fair opened, he said. Ride operators are supposed to do three daily operational checks and record those in a log, he said. From July 1, 2011, to June 30, 2012, there were 11 accidents on amusement rides in the state and of those, all of them were "patron error" -- meaning it was the customer's fault, O'Briant said. Television station WRAL in Raleigh quoted witnesses as saying several people were thrown off the ride and that some were unconscious. A witness identified as Caleb Norris told WNCN television that he heard a crashing sound just after getting off the Vortex. He turned around and saw two people lying face down. Norris also said he saw the ride operator fall to his knees and start crying. The accident occurred shortly before the fair was shutting down for the night. Long said the fair would reopen on Friday as scheduled. It closes Sunday. In 2002, a ride operator at the fair was killed when he was struck by the ride while it was still in operation.