Deputy police commissioner Nahidul Islam told AFP news agency that Siddique had been involved in cultural programmes, and had set up a school at Bagmara, a former bastion of outlawed Islamist group Jamayetul Mujahideen Bangladesh (JMB).
Members of JMB were arrested over an assault on an Italian Catholic priest late last year.
Earlier this month a Bangladeshi law student who had expressed secular views online died when he was hacked with machetes and then shot in the capital, Dhaka.
The four bloggers killed last year had all appeared on a list of 84 "atheist bloggers" drawn up by Islamic groups in 2013 and widely circulated.
There have also been attacks on members of religious minorities including Shia, Sufi and Ahmadi Muslims, Christians and Hindus. And two foreigners, an Italian aid worker and a Japanese man, were also shot dead late last year, in seemingly random attacks.
The so-called Islamic State group has said it carried out many of the attacks - but this has not been independently verified.
Bangladesh is officially secular but critics say the government has failed to properly address the attacks.