Salwinder Singh, the former Gurdaspur SP who is being probed for any possible link with the Pathankot airbase attackers, has been "cooperating well" with the National Investigation Agency (NIA), official sources told IANS on Monday.

According to the sources, the investigators have so far found nothing "incriminating" that could link Salwinder with the Pathankot terror strike.

Questions are being raised why the terrorists let Salwinder off and not harmed him. There were also suspicions about his involvement with a drug smuggling cartel.

However, the NIA is not specifically investigating the drug angle since that would be outside its purview. If any facts connected to the smuggling come to light, the matter would be handed over to the appropriate agency for probing, the sources said.

A source in the ministry of home affairs told IANS on condition of anonymity that Salwinder had told investigators that the terrorists let him go because he was a Sikh.""This is not unbelievable as the Inter-Services Intelligence (ISI) of Pakistan has been trying to stoke the Khalistan movement in the north Indian state of Punjab. The ISI may have specifically briefed the terrorists not to harm any Sikh," the source said.

Meanwhile, the investigators also got permission from a Delhi court on Monday to conduct a lie detector test on Singh. The test, the source added, was being done to be doubly sure of the facts that Salwinder had mentioned in his interrogation.

Six Pakistani terrorists -- believed to be from the outlawed Jaish-e-Mohammed terror outfit -- sneaked into Punjab from across the border and took the Punjab Police officer hostage before taking away his vehicle. Later, they raided the IAF base at Pathankot, killing seven security personnel.