Ten undertrials escaped when they were being taken for a court hearing from the high security Naini Jail in Allahabad, on Monday afternoon.
An initial probe has revealed that the rear door of the van was not locked.
Inspector general of police (law and order) Raj Kumar Vishwakarma told IANS that while one sub-inspector and eight cops were listed on the duty muster, four did not report for duty.
The other four who reported on duty were sitting in the front cabin of the prison van with the driver.
"A probe has been ordered into the incident. We are trying to ascertain the policemen did not report for duty and why were the undertrials left unescorted and the door of the jail van was open," said a senior police official.
"A major operation is on to trace the absconding prisoners and action has been ordered against the errant cops," Vishwakarma informed.
The prisoners who escaped are Rizvan, Rakesh Chowdhary, Chand Babu, Shailesh Kumar, Rajesh Yadav, Anil Vishwakarma, Jai Prakash, Atul Kumar Pandey, Mohammad Vaish and Vijay Yadav.