One or two of the inmates fell ill near Weatherford, which is about an hour west of Oklahoma City on Interstate 40, Police Chief Byron Cox said. Assistant Police Chief Louis Flowers said the inmates had been left in a locked compartment inside the van.
"Two of the prisoners kicked the partition out and were able to gain access to the front of the van. ... The keys were still in the van as far as we know, and it was still running," Flowers said.
Cox said the inmates stopped about one mile down the road and two ran away. Dogs and helicopters were summoned to aid in the search, and officers went door-to-door during the afternoon, Flowers said.
Oklahoma Highway Patrol spokeswoman Betsy Randolph said officers recovered a 12-gauge shotgun that had been inside the vehicle. No injuries were reported.
No details were available about what the inmates may have been accused or convicted of doing.
Prisoner Transportation Services of Nashville, Tenn., acknowledged it was moving the prisoners but said it could not comment further because the investigation was ongoing.
Critics of private prison transportation companies have complained that they are poorly regulated.
Southwestern Oklahoma State University in Weatherford was locked down for about two hours while police searched for the two inmates. The school lifted the lock down just before 4 p.m., KOKH television reported.