DHAKA, Bangladesh — A court in Bangladesh sentenced to death five members of an Islamist militant group on Tuesday for killing an American blogger critical of religious extremism six years ago.

       Avijit Roy, a United States citizen of Bangladeshi origin, was hacked to death by machete-wielding assailants in February 2015 while returning home with his wife from a Dhaka book fair. His wife and fellow blogger Rafida Ahmed suffered head injuries and lost a finger.

       “Charges against them were proved beyond any doubt. The court gave them the highest punishment,” the public prosecutor Golam Sarwar Khan said after the verdict, amid tight security at the Special Anti-Terrorism Tribunal in the capital, Dhaka.

       The court also jailed one man for life in the attack, Mr. Khan said.

       He said the six men convicted on Tuesday belong to the al Qaeda-inspired domestic militant group Ansar Ullah Bangla Team, which the police say was behind the murders of more than a dozen secular activists and bloggers.

       Syed Ziaul Haq, a sacked army major believed to be the leader of the group and who masterminded the killing, and one other member of the group were tried in absentia and received death sentences, Mr. Khan said.

       Nazrul Islam, a defense lawyer for the six men, said they would appeal against the sentences in the higher court.

       Muslim-majority Bangladesh saw a string of deadly attacks between 2013 and 2016 targeting bloggers, secular activists and religious minorities, claimed by Islamic State or al Qaeda-aligned groups.

       The most serious attack came in July 2016, when gunmen stormed a cafe in the diplomatic quarter of Dhaka and killed 22 people, most of them foreigners.

       After the cafe siege, more than 100 suspected militants were killed and hundreds more were arrested as the government cracked down on Islamist groups to preserve its image as a moderate Muslim nation.