Each cell operates under a terrorist group or organisation.
It is only when a blast takes place that police come to know about the sleeper cell that was quietly providing the inputs over months, or maybe years.
Jaish-e-Mohammad, the Kashmir-based terrorist group which was involved in the attack on the Indian parliament, had set up its sleeper cell in the city ahead of the attack.
"The five Pakistani terrorists who were killed by Indian security personnel during the attack on parliament had been given inputs for the operation by members of such sleeper cells," senior police officer Ashok Chand, the then Deputy Commissioner of Police (Special Cell), told IANS.
