CBI sources said the tapes provided by one of the accused Gujarat police officer to the agency have not been edited and prima-facie appear to be genuine.
They said that in order to understand whether the voices in the tapes are of those to whom they have been attributed, the agency would require court's intervention. There are people in the conversation which are purportedly Gujarat government officials identified by some witnesses and accused during their questioning, they said.
Any comparison of the voices in the tapes with the suspected participants can only be done with the voice samples which can be taken only after court's permission, they said.
The sources said the person who is approached with the request for the voice sample has to give his consent which is mandatory. They, however, claimed that these tapes are legally admissible evidence in the court. Mostly these conversations relate to SIT investigation and planning in its aftermath.
The sources said the agency is likely to use the conversations in the tapes, which have already been submitted to trial court in Ahmedabad in sealed covered, to give details of the conspiracy which led to the encounter nine years ago.
They said the agency is working on its supplementary charge sheet which might name some of the intelligence officers as accused and also the larger conspiracy behind the encounter in which 19-year-old Ishrat and three others -- Javed Sheikh, Amjad Ali Rana and Zeeshan Johar --were killed in June, 2004.
It has been able to get detailed information about Ishrat and Javed as it included the statements of their relatives and people known to them in the charge sheet filed on July 3.
The agency is also working on to get the antecedents of Johar and Rana and announced reward in Kashmir newspapers to anyone who can give information about their background.
CBI has in its charge sheet neither given any details about the antecendents of the duo nor the motive of their presence in Gujarat.
CBI accused seven police officers of Gujarat Police of murder, kidnapping and other related crimes under the Indian Penal Code.