DHAKA : Gunmen opened fire and tossed bombs on Sunday at a prison van carrying detained militants of a banned Islamist movement to a court house, killing a policeman and freeing three activists, police and witnesses said.Police said two cars blocked a highway in Mymenshing district, 122 km (76 miles) north of Dhaka, and set off several bombs. One policeman died on the spot and three were injured.The three convicts snatched from their police escort were members of the banned group Jamaat-ul-Mujahideen Bangladesh (JMB), including the group’s explosives expert. Two of them had been sentenced to death.They had been on their way to a court in Mymenshing to testify in another trial, said Abdur Razzak, superintendent of Kashimpur jail, where all three had been held. Police mounted a manhunt and were combing districts near the incident.Six leaders of the Islamist militant group were hanged after being convicted of killing two judges in 2005. Among them was JMB chief Shaikh Abdur Rahman, a veteran of conflicts in Afghanistan.The group exploded nearly 500 bombs almost simultaneously on Aug. 17, 2005 across Bangladesh, including the capital Dhaka, in attacks largely aimed at frightening authorities.Its militants later carried out suicide attacks on various courthouses, killing 25 people, including judges, lawyers and policemen, and injuring hundreds.