COLOMBO, Sri Lanka—Islamic State claimed responsibility for the Easter bombings that killed more than 300 people in Sri Lanka and posted a video that it said showed the suicide bombers, dressed in black robes, declaring their loyalty to the group.

“The executors of the attack that targeted citizens of coalition states and Christians in Sri Lanka two days ago were Islamic State fighters,” the group, also known as ISIS, said Tuesday, according to SITE, a security-consulting firm that tracks militant organizations.

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