Authorities found the bodies of a husband and wife at the Lohi Bheer Police Foundation and those of a 10-year-old child, a woman aged about 23 or 24 and two men in their mid-30s near the city's Sahala station, The Express Tribune reported.
The Sahala victims had been killed at a different site and dumped near station with their hands tied, investigators said.
In a third incident believed to be unrelated, the body of a Pakistani-born girl from Singapore, kidnapped three days earlier, was found, police said, adding a ransom of 20 million Pakistani rupees ($188,240) had been demanded.
The television news channel Dawn News, based in Karachi, said Islamabad has seen an increase in extortion, kidnapping for ransom and bank robberies.
Police say an outlawed terrorist group, Tekrik-i-Taliban Pakistan, is behind the crimes, extorting money from wealthy residents and encouraging smaller gangs to collect extortions and share its profits with the group.