Shafiur Rahman Farabi is a religious extremist who has been rather open about his business. His numerous postings online show his main mission is to identify free-thinking bloggers and hound them on social media platforms.

This is exactly what happened in the case of slain writer-blogger Avijit Roy. Farabi became a Facebook friend of Avijit about five years ago. Later, he started reading Avijit's blog, Mukto-Mona, and came to learn about his views on religion and science.

Farabi has been "very active” in spreading fanatic ideologies online since he joined the Hizb ut-Tahrir in 2010, a year after the militant outfit was banned, when he was

student of physics in Chittagong University, law enforcers and campus sources said.

The 29-year-old might have been covertly involved in militant activities even before that, they added.

Farabi was first arrested in 2010 for vandalism on the university campus in August that year. Having walked out of prison a month later, he started writing on blogs and social media platforms about Islam and how it should be practised. His blog became an open hub for sharing of fanatical thoughts as he began to counter free-thinking bloggers, including Avijit.

His blog and several Facebook accounts show a large following, mostly radical in their thinking.

Police detained him for a second time from Chittagong University campus in February 2013 for threatening to kill the imam who would administer the funeral prayers of young blogger-architect Ahmed Rajib Haider, who was killed allegedly by militants in the capital's Mirpur that year.

A Dhaka court framed charges against him in this case in June 2013. The charges carry maximum 14 years in jail and a fine ofTk 1 crore.

During his time in Kashimpur jail in this case, Farabi became "very close" with Ansarullah Bangla Team's spiritual guru Jasimuddin Rahmania and his aides, intelligence sources said.

This militant outfit was found to be involved in Rajib's murder, and Jasimuddin and six North South University students linked to this group were put in jail.

Farabi came out of prison on High Court bail in August 2013, only to step up his activities, publicly threatening to kill different people, including Avijit.

On August 21 that year, he took to the Facebook to demand the release of the North South University students who confessed their involvement in the killing of Rajib.

In one post soon after his release, he wrote: "To me atheists are nothing but insects, and it is best for insects to die."

Law enforcers say they are aware of Farabi's death threats and provocative activities online, but it remains a mystery why he was not arrested for those.

Farabi would identify and develop relations with progressive bloggers and ask their phone numbers so they could discuss their faiths. This way he came in contact with many and learnt their details, according to online activists.

Detectives believe he has a strong link with Jasimuddin and his Ansarullah Bangla Team, although they are not sure who his mentor is or which group he is currently involved with.

Born in 1986 in Kalaisri Para of Brahmanbaria, Farabi got admitted to CU in the academic session 2005-06 but is yet to complete his education as he failed his fourth-year exam several times, campus sources said.

In his early life, he went to Railway Govt Primary School in Kishoreganj and to Kendua Joyhori Sky Govt High School. He did his HSC from Notre Dame College in Dhaka in 2005.