Mohammad Eisa Fazli, a resident of Shadab colony near Soura in the outskirts of Srinagar, went missing from his hostel room in Baba Ghulam Shah Badshah University (BGSBU) in Rajouri on August 17.
Two days later a picture of him holding the rifle was uploaded on Facebook announcing that he had joined "jihad" (holy war).
He had joined B Tech (IT) course in the university in 2014.
The police was trying to ascertain how the student got radicalised and left his studies after completing his sixth semester to join the militant group.
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"The boy had gone to his home for a two-month holiday and is believed to have come in contact with militants there. Srinagar Police is investigating the matter," Senior Superintendent of Police, Rajouri, Yougal Manhas told PTI.
"As part of our investigation, we visited the university, conducted a search of his room and questioned his close friends and roommates. But nothing incriminating was found from his room," the SSP said.
"We came to know that he had talked about Azadi (freedom) of Kashmir on several occasions but never expressed his desire to join militants," the officer said.
He said there are two types ofradicalisation--expressed radicalisation and internal radicalisation--and his case was an example of internal radicalisation.
Manhassaid police has not come across any case of expressed radicalisation in the ongoing investigation so far.
Training and placement officer Sameer Raina said the university had been holding counselling sessions with the students from a long time to keep them away from militancy.
He said the report of his joining militant ranks was on social media and they could not be sure that the news was "authentic but at the same time we cannot ignore such a development".
The case had shocked the entire university and the matter was brought to the notice of police and civil administration, he said.
"The student was gentle and not a talkative one. He was good in academics and above average. He cleared his sixth semester exam on July 18 but did not join the seventh semester even after returning from holiday on time," Raina said.
In a statement, the university said the parents of the student were informed about his leaving the hostel without permission and asked to know his whereabouts and safety.
However, no information was received from the parents, the statement read.