NDTV has accessed the audio messages - 20 of them in Malayalam were sent on WhatsApp and the Telegram app, which destroys a message within seconds of it being seen.
Investigators said most people who were added to the WhatsApp groups quickly left them and so did not access the messages. Others have reported them to investigators. The worry, however, is that there could be those who have not reported receiving these messages, which indicate continued efforts to radicalize and recruit Indians for the ISIS.
Sources in the elite National Investigation Agency or NIA say that the main voice in the audio clips is that of Rasheed, a 30-year-old engineer who left a private sector job to teach in Kerala's Kasaragod before leaving the country in 2016, and is believed to be the man responsible for indoctrinating the 21 people who have left Kerala since last year to join the ISIS. Rasheed is believed to be hiding in Afghanistan.
In another, "It's not about peaceful prayers, it's Jihad that is needed...unless you are jihadis, you cannot increase Islam; take up the sword...even small children of 3 or 4 uses their toy guns."
The NIA says 21 people, including pregnant women and children have travelled out of the country from Kerala since last year to join the ISIS. Two men are believed to have died in US drone attacks and one of the audio messages confirms that.
The hunt for Rasheed started after the NIA arrested six men from Kerala in October, who were allegedly planning terror attacks in the country.