Moyna Mia, 45, was nabbed at Tuker Bazar, on the fringe of Sylhet City, with help from the locals around 10:30pm on Tuesday, said Jalalabad Police Station OC Akter Hossain.
The man is also known as ‘Boro Moyna’ at Kumargaon village, where he and three others tortured Rajan to death on July 8.
The other three accused in the case filed by police are Muhit Alam, Kamrul Islam and Haider Ali.
Of them, only Ali is still absconding.
Rajan’s uncle Omar Faruk told reporters that many in their village alleged that Moyna Mia was a ‘paedophile’. “He particularly violated minor boys.”
Family members earlier had alleged that Moyna wanted to sexually assault Rajan.
But after failing to have his way, the man staged a drama of theft to kill the 13-year old boy, they claimed.
Locals caught Muhit Alam on July 8 when he was trying to dump Rajan’s body. Later he was handed over to police.
Alam’s brother Kamrul Islam was taken into police custody in Jeddah by local law-enforcers on Monday after the Bangladesh embassy in the Saudi Arabian city sought their help.
Police in the past two days also detained Alam’s wife, brother-in-law, and two others who witnessed the gory torture on the boy for questioning.
Alam and another suspect are currently on police remand in the murder case.
Earlier, Home Minister Asaduzzaman Khan Kamal had ordered police to arrest all the accused as soon as possible.
Inspector General of Police AKM Shahidul Hoque had said they were doing their best.
Meanwhile, Jalalabad police Sub-Inspector Aminul Islam was withdrawn from duty on Tuesday after he was found guilty of misconduct with Rajan’s father.
Islam was the officer who had filed the case.
Sylhet Metropolitan Police’s Additional Deputy Commissioner Rahmat Ullah told bdnews24.com that SI Islam was withdrawn and sent to the district’s Police Lines after the probe team found proof of his alleged misbehaviour with the boy’s father.
Several men on suspicion of theft barbarically tortured Rajan to death on July 8 at Kumargaon village.
One of the suspected assailants had filmed the whole incident and later released it on the internet. The 28-minute video clip created a furore on the social media as it went viral.
Since then, different organisations have been demonstrating across Bangladesh demanding maximum punishment to the culprits for their heinous crime.