Police arrested suspended sub-inspector Akbar Hossain Bhuiyan in connection with the custodial death of Raihan.
A team of Sylhet district police arrested him from Dona area along the border of Kanaighat in Sylhet on Monday.
District additional superintendent of police Lutfar Rahman confirmed this to Prothom Alo.
He said Akbar was arrested while he was fleeing to India.
Akbar was in charge of Bandarbazar police line in Sylhet.
A source said Akbar was staying in Khasia Palli on Indian side of the border with Dona in Sylhet's Kanaighat. Khasia community tactfully sent Akbar back to Bangladesh and, being informed, police arrested him.
Speaking to Prothom Alo, two witnesses along Dona border area said Akbar introduced himself during his arrest. He dressed in clothes of the Khasia community.
Raihan, a resident of Niharipara of Akhalia in Sylhet city, was picked up and tortured at the Bandarbazar police line in Sylhet on 10 October. He died on the following day.
His wife Tahmina Aktar filed a case with Kotwali police station in connection with the death under police custody.
A police investigation team found the allegations of torture true.
On 12 October, four policemen including SI Akbar were suspended and three others withdrawn. Akbar fled on 13 October while he was in police custody.
At the directives of police headquarters, the investigation of the case was shifted to the Police Bureau of Investigation (PBI). Three constables described the incidents and mentioned the names of the torturers at the court on 19 October.
SI Akbar was the mastermind in the torture of Raihan.