Police on Wednesday arrested one of the accused in connection with the killing of Sharmin Akter Kakoli, a ninth-grader, at Islamabad village in Matlab Uttar upazila.
The arrested, aged 15, hails from Sujatpur village in the upazila. He is the classmate and former boyfriend of Kakoli.
Matlab Uttar police station Officer-in-Charge Md Nasir Uddin Mridha confrimed the matter on Friday.
Nasir Uddin Mridha said Kakoli had a love affair with both her classmate Saif and a boy from Rajshahi.
The two boys made a plan to take revenge as Kakoli was continuing relationship with both of them at the same time.
On March 28, the former boyfriend made a phone call asking Kakoli to meet him at Oxford Academy where Kakoli's new boyfriend was present.
The two boys took Kakoli to an isolated place near the school building where they raped her in turns and strangled her, the OC said.
Later, they slaughtered Kakoli and dumped the head in a nearby drain. After the incident, the new boyfriend went into hiding but the former one stayed at his residence.
Police recovered the body of the schoolgirl from a classroom of Oxford Academy on Wednesday and later the severed head from a drain on Friday.
