Six people have been charged with the gruesome murder four years ago of a controversial atheist blogger, Bangladesh police said Monday.Avijit Roy, a Bangladesh-born US national, was hacked to death on a busy road outside Dhaka University in February 2015. His wife was critically injured in the attack.Police investigators said 11 of the 12 militants from the outlawed Ansar al Islam group implicated in the murder took part in the street killing. Six people have been charged over the murder of Avijit Roy. Of the six, two are on the run, Dhaka police spokesman Masudur Rahman told AFP.A sacked army major who masterminded the attack was one of those still on the loose, Rahman said, adding police could not identify the other five involved.One suspect was killed by police in a crackdown that followed the murder. Roy, who migrated to the United States two decades ago, was known in Bangladesh for his Mukto-Mona (Free-mind) blog, where he railed against all forms of organised religion.One of his books, The Virus of Faith, became a controversial bestseller in Bangladesh, which is officially secular even though 90 per cent of the population is Muslim.Ansar al Islam has been accused of the 2013-14 murders of atheist and secular bloggers and gay activists that prompted many to flee the country.The government launched a crackdown on Ansar and other radical groups after an attack on a Dhaka cafe in July 2016 left 18 foreigners dead.Nearly 100 extremists have since been killed by police while hundreds have been arrested.