Nur Mia was arrested on the outskirts of the northeastern city of Sylhet late yesterday, becoming the eighth person to be held over the July 8 killing of the boy, who was tied to a pole and then subjected to a brutal assault in which he pleaded for his life.
Police said they found the second footage of the incident in Mia's mobile phone.
Meanwhile, authorities ordered the detective branch of the police to take charges of the probe into Sheikh Samiul Alam Rajon's brutal muder.
The development came as a fresh two-minute video capturing the pictures of the boy's torture was circulated on social media.
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"Beat him, beat him in any method you want...Let us beat him in competition," Moyna Mia, a key-suspect in the case who was also detained yesterday, was heard as saying in the video.
Autopsy reports said the body carried 64 injury marks but the boy died of brain hemorrhage caused by torture.