The Central Bureau of Investigation on Friday began questioning the main accused in the Nithari serial killings -- Moninder Singh Pandher and Surendra Koli -- as the agency dug a stretch of drain where skeletal remains of killed children were exhumed.

As 16-member forensic expert team along with CBI sleuths visited the spot and started examining the crime scene and collecting evidences, including digging up the drain behind Pandher's house, the entire process was being videographed for investigation purposes.

The CBI, which took custody of Pandher and Koli on Thursday, began questioning them extensively and did not rule out the possibility of subjecting the duo to polygraphic test, an exercise that could be used as a circumstantial evidence in the case.

CBI Director Vijay Shanker along with his Special Director M L Sharma visited Nithari to make an on-the-spot assessment and give directions in the case.

"We have attached high priority to the case and therefore it is necessary to have a feel of the location," Shanker said after his visit to the spot.

Avoiding media glare, he dumped his official car with beacon light and boarded a private taxi.

"This case is important for us and it will be solved by a combination of good investigation and appropriate medical and forensic backup," Shanker said

The team, comprising nearly 30 members, also inspected adjoining buildings including D-6, the residence of Naveen Chaudhary, a doctor whose name figured in a organ trade racket in 1999.

The CBI's Special Crime Branch, which is investigating the shocking case, has formed over a dozen teams to probe every aspect of the grisly crime and look into whether there were any lapses on part of the local police.

The CBI has roped in forensic experts from the All India Institute of Medical Sciences, Andhra Pradesh Forensic Laboratory and Central Forensic and Sciences Laboratory to conduct tests on the bones.

The forensic experts who visited the scene of crime are K L Dogra from AIIMS, K P Gandhi from Andhra Pradesh Forensic Laboratory and S R Singh from CFSL. The agency sleuths might also question staff at nearby hospitals and nursing homes amidst reports that the Nithari killings were linked to a human organ racket, sources said.

The CBI team, led by DIGs M K Narayanan and Arun Kumar, was accompanied by senior Noida police officials including SSP R K S Rathore. The CBI has also called suspended Noida Superintendent of Police Saumitra Yadav for recording his statement as part of their probe.