The accused allegedly used to obtain the CDRs illegally, a police official said.
The police was on the lookout of one more person allegedly involved in the racket, he said.
As per regulations, only those authorised, such as police and some other departments, were permitted to access call data records with prior sanction from the authorities concerned, the official said.
The CDRs were generally accessed in order to be used as evidence in courts in cases of crime and other offences, he said.
Acting on a tip-off, a team laid a trap at a park in the Kalwa township and nabbed the four accused on January 24, Thane's Deputy Commissioner of Police (Crime) Abhishek Trimukhe told reporters in Thane on Wednesday.
Those arrested were identified as Makesh Madhavan Pandian, 42, who ran the detective agency in Navi Mumbai, and his associates Prashant Palekar, 49, Jigar Makhwana, 35 and Samresh Nantun Jha alias Pratik Mohpal, 32, police said.
The accused apparently sourced the CDRs from some people in neighbouring Mumbai and used to illegally sell it to agencies and individuals who asked for it, he said.
They were booked on charges of cheating, and several sections of the Information Technology Act, the official said.
An investigation was on into the case, he added.
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