Arun Selvarajan's mobile phone conversations were intercepted for weeks by the NIA sleuths and he was kept under "high-tech surveillance."
After several weeks of monitoring, it became clear that his intentions were "illegal, anti-national and intended to aid an inimical neighbour's nefarious designs," an official said.
The intercepts gave ample clues that Pakistan was now "seriously working" to set up modules in South India to target "high value assets," through "new avenues."
"Several bank accounts,the numerous dubious transactions, and money transfers strengthens the case."
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The eventual arrest of the Sri lankan began with efforts to "join dots to fix some missing links" in the Thameem Ansari spying case which is also being probed by the NIA.
Sources, however, declined to specify if Thameem Ansari and Arun Selvarajan knew each other and whether they were aware that they were working for the "same boss."
He was arrested on September 10 and remanded to judicial custody the following day.
Laptop, hard disks, pen drives, micro-tape cassettes were among the seizures made by the NIA from his residence.
Also, the accused was found in possession of both Indian and Srilankan passports besides ganja.