And for those two months, they remained a mystery. They used mobile phones to threaten their targets and shot at those who didn't pay up. The plan was perfect and it worked -- till last week when the police closed in on them.The extortionists -- Dalip Kumar, Mahender, Dara Singh and Ashok Kumar landed in the police's net. But tracking them down was not so easy.Dalip was the mastermind. And he was more than familiar with the police's methods of investigation.

Investigators had been tapping their cellphones, takingprintouts of the outgoing and incoming calls and monitoring their conversation. The group had four mobile phones but were using only two for making threat calls. The other two were used for their personal communication which meant that only two mobile numbers were on the watchlist of the police.

Every cellphone has a instrument-specific code number which automatically gets registered into the computer of the cellphone company the moment a cash card is inserted in it. The breakthrough came when Dalip Kumar disposed of his mobile phone and bought a new instrument with a new cash card.

But he handed over his old cash card the one used for making threat calls to one of his accomplices, Mahender. He inserted the cash card given by Dalip in his phone and made a few threat calls. So the instrument's code number got registered in the cellphone company's computer and since the police was monitoring this cash card number, he too joined the list of suspects.

On the intervening night of March 20, joint teams ofGurgaon and Delhi police launched `Operation X-ray'.

Around 200 armed police personnel swooped down on 22 places in Gurgaon and Delhi -- from where the extortionist had made calls from his personal cellphone.

The cops questioned around 50-60 persons and zeroed in on five men before arresting four of them in connection with the extortion racket.

Incriminating evidence such as a telephone diary with the numbers of persons to whom the threat calls were made, cellphones, weapons and cash over Rs 3.5 lakh were recovered from the extortionists.

Senior Superintendent of Police, Navdeep Singh Virk said that two prominent doctors Dr Lal and Dr Aryan who had received extortion threats had actually paid Rs 2.50 lakhs and Rs 5 lakhs respectively to the extortionists. DSP Anil KUmar Yadav said that during the interrogation, Dalip Kumar revealed that he got the idea to form an extortion gang after watching the movie Satya.

The accused, meanwhile, have been remanded to police custody till March 27.