Aarushi case: CBI to take help of foreign forensic laboratory

The samples including the bedsheet, blood, fingerprints, bottles and her vaginal swab were all contaminated.

The CBI has contacted a forensic laboratory outside India where certain samples collected from the scene of crime will be sent in order to extract DNA from them as it would help further investigations in the case, sources privy to the probe said.

The laboratory has the expertise for extracting DNA from samples which are highly contaminated but have been touched by the criminal. The technique used is called touch DNA which means that the DNA of the person can be obtained from an object that he has touched. It has helped solve dead-end cases, the sources said.

When any individual touches an object he leaves behind a print that is basically made up of oils and dirt that are constantly deposited on his fingers.

As such everything that is touched picks up a bit of oil and dirt which are left as fingerprint on the object. Body cells become part of the fingerprint and it is these that are the source of the DNA.

A single cell is enough to provide enough DNA for analysis. So if the murderer has touched anything in the house, his DNA profile can be chalked out even if he has used gloves because skin cells keep on drying and falling from the body, the sources said.

The samples that were collected from Aarushi's house were declared to be highly contaminated by a forensic laboratory here. DNA evidence can become contaminated when DNA from another source gets mixed with DNA relevant to the case. "As such no forensic evidence could be extracted by which we could zero in or rule out those suspected in this murder case," he said.

"This laboratory has been chosen after visiting nearly all websites of the foreign laboratories that have this technique," the sources said. Aarushi, 14-year-old daughter of a successful dentist couple Dr Rajesh and Nupur Talwar, was found dead with her throat slit in her parents' home at Jalvayu Vihar in Noida on May 16, 2008. So far, the police and later the CBI failed to even identify the murderer.