The U.S. Department of State’s Diplomatic Security Service is offering a reward of up to $5 million for information related to a terrorist attack in Dhaka, Bangladesh that left influential blogger Avijit Roy dead and his wife, Rafida Bonya Ahmed, seriously injured.

Coordinated through the Secretary of State’s Rewards for Justice (RFJ) office, the reward is for information leading to the arrest or conviction of anyone involved in the 2015 murder, a State Department press release said.

“This investigation remains open, and we are seeking information that will assist law enforcement agencies in bringing to justice the perpetrators of this heinous terrorist attack,” the release said.

The Bangladesh-born U.S. citizens were visiting Dhaka in February 2015 for a book fair when they were attacked by assailants with machetes, the release said. Roy died at a nearby hospital, while his wife suffered several injuries to her body, including a severed finger, a report by CNN said.

Roy was a prominent blogger and outspoken critic of religious extremism, CNN reported. He wrote in a Free Inquiry article in 2014 that an Islamist activist said he “will be murdered when he comes back” to Bangladesh.

To date, six individuals have been tried and convicted, including conspirators Syed Ziaul Haque (aka Major Zia) and Akram Hussain, both of whom were tried in absentia and remain at large, the release said.

Two related groups have claimed responsibility, including al-Qaida in the Indian Subcontinent (AQIS), which the State Department recently designated as a Foreign Terrorist Organization, the release said. The group is also a Specially Designated Global Terrorist under Executive Order 13224, “which provides authority to sanction terrorists and those who support terrorists or terrorist acts.”

RFJ has awarded more than $200 million to individuals who have provided information leading to the arrest of terrorists and the prevention of attacks, the release said. The program was established in 1984.