The four alleged contract killers, identified as Robinson, Junaid, Yunus and Manish, were arrested on June 3 following which they were sent to police remand and interrogated for five days.
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Later, they were produced in a court which sent them to judicial custody, a senior police official said today.
"Investigation is underway," Special Commissioner of Police (Special Cell) Arvind Deep said.
Delhi Police's Special Cell zeroed down on the four through telephone intercepts. The accused were in constant touch with Shakeel, police claimed.
Once identified, the four were picked up from their residences in Rohini in Outer Delhi, Seelampur in northeast Delhi, Ghaziabad and Noida, the official said.
Police also claimed have recovered a 9mm pistol and live cartridges from possession of one of the accused.
The four were in direct touch with Shakeel through Internet telephony and phone. The idea was first to kill Rajan's trusted lieutenant, his driver, who's coming to Mumbai to meet him in jail. They had planned to eliminate Rajan while the don is taken to court for hearing.
The four accused are also lodged in Tihar jail, where Rajan is in a high-security ward, the official added. The Special Cell has written to Tihar to keep a tab on suspects movements.
Rajan (55), who was on a run for around 27 years, was arrested from Bali in Indonesia, based on a tip-off from Australian Federal Police, and brought to India in November last year.