Singh, at present posted as Assistant Commandant of 75th Punjab Armed Police after being shunted out as Superintendent of Police (Headquarters) Gurdaspur, had agreed for a polygraph test after NIA informed a designated court about alleged "inconsistencies" in his statements before Punjab police and NIA.
He was being questioned by a team of experts from Central Forensic and Scientific Laboratory and the questioning will continue on Wednesday, official sources said.
They said Singh was being questioned to ascertain the sequence of events that took place after he was kidnapped on the intervening night of December 31 and January 1 by terrorists of Pakistan-based Jaish-e-Mohammed.
Eyebrows were raised when Singh said that he and the cook were released whereas one of their friends, who was travelling with them -- Rajesh Verma -- had been left bleeding midway by the terrorists.
"We are only looking for correct sequence of events so that the investigations can progress in right direction," a NIA official said.
Also his statement that he was returning from a shrine, which he often visited, was found to be allegedly incorrect after NIA questioned caretaker of the dargah Somraj who told the probe agency that the police officer had come for the first time.
Terrorists had struck at the IAF base on the intervening night of January 1 and 2 in which seven security personnel were killed in the encounter that lasted for three days.
So far four bodies of terrorists have been recovered while two others are believed to have been burnt in the building where they were holding for more than 10 hours.
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