The 'closeness' of Anurag Singh, the man accused of accessing the call details of leader of opposition in Rajya Sabha Arun Jaitley, with the Delhi Police's special cell is proving costly for the elite unit.

Touted as a cyber expert who had helped the police in several high-profile cases - the 2001 Parliament attack case, an MMS scandal in a private school and a match-fixing scandal are some - Singh has been able to confuse investigators since he is aware of the working style of the special cell.

Investigators said he has the call records he had accessed in his phone but it is password-protected and he isn't revealing the details. "His phone contains crucial details and we are now engaging another cyber expert to extract the information," a senior police officer said.

So far, the police have got to know that the IDs and passwords of four assistant commissioners of police (ACPs) were used to get call details of 13 people, including Jaitley and several businessmen. Sources said Singh got call details of prospective brides and grooms on the demand of his clients and charged them a hefty amount for it.

The special cell had called the four ACPs for questioning and the inquiry revealed that IDs of only three were used illegally, while one ACP had officially requested to obtain call details of Jaitley after the arrest of Sukhdev Singh Namdhari in the Ponty Chadha shootout case.

"The ACP was given a clean chit. The investigation of the shootout was transferred to the crime branch and the Special Investigation Team (SIT) had sought details of Jaitley's call records from the telecom operator in connection with that case," the officer said.

To avoid any embarrassment in future, the Delhi Police have directed ACPs not to share their passwords with anyone or end up losing their jobs if they do so.