The arrest was announced in a press conference by Sukkur AIG Dr Kamran Fazal, Sukkur DIG Fida Hussain Mastoi and Khairpur SSP Captain (retd) Ameer Saud Magsi.
AIG Fazal said they conducted geo-fencing of tens of hundreds of cell phones apart from taking DNA samples.
The police officer said the accused has confessed to the murder; he victimised the child to avenge her sister's death who was killed by the victim’s uncle Khadim Larik on karo kari.
The arrested man identified and disclosed the whereabouts of the other co-accused, Salim and Mansoor Shar, the police said.
Later, Sindh Inspector-General of Police Mushtaq Ahmad Mahar visited the victim's family to condole with her parents. The Sukkur AIG and DIG and Khairpur SSP accompanied him.
The IGP expressed his shock and anguish over the tragedy and informed them about the arrest of the prime accused and search for co-accused. He also announced rewards for officers of the Khairpur police who arrested the culprits.
Earlier on Monday, police said that DNA of a suspect in the Khairpur rape and murder case matched the victim's sample.
The seven-year-old was strangled after being raped on January 11. The DNA tests were conducted on more than 365 suspects and a match was found with the victim's relative. Teams have been made to arrest the suspect, police said.
Last week, police arrested five suspects in the case. A case has been registered against three unidentified persons on the complaint of the child's father, the police had said.
The seven-year-old girl was raped and murdered after being abducted in Pir Jo Goth city of Khairpur district. Police had told media that the minor girl had been subjected to rape before being murdered. The body of the girl was found from a forest by the police officials. The incident created panic and fear among the locals, who protested the latest gruesome incident of child abuse and demanded that the rapist be arrested. TLTP