Police in the Nigerian city of Jos are investigating the circumstances of how a 15-year-old boy staged his own kidnap and demanded a ransom from his father for his release. "It was expertly done," police spokesman Terna Tyopev told the BBC. The teenager and four of his older friends, who are now all in custody, demanded a 500,000 naira ($1,365, £1,126) ransom, he said. They had planned to use the money for a school-leaving graduation party. Three days before the staged kidnap, the boy's father had sold his car to raise money for the family's upkeep. The 15-year-old came up with his plan after searching the family home and failing to find the cash from the car sale. "The money had been paid into the father's account, so it was not there," Mr Tyopev said. So the friends pretended to be the kidnappers and contacted his father. "They warned him the son had been taken far away from Jos and he should not report to security agencies. The father reported only after the threat became very serious," the police spokesman said. The alleged abduction was foiled when officers tracked down the phone number used to make the ransom calls to an apartment in the city. The teenager and his friends were having a conversation when police stormed the building. "The father felt very terrible at the discovery. His son had not asked him for money for the party beforehand," Mr Tyopev said. Although the plan was the son's idea, the expert execution of it by his friends - who are aged between 18 and 22 - has made officers suspicious. Police were now investigating if they had abducted people for ransom in the past, Mr Tyopev said. Kidnapping for ransom is rampant in Nigeria. Gangs nationwide kidnap both rich and poor people, often collecting ransoms of up to $150,000 - and sometimes killing abductees whose families fail to pay. Police chief Mohammed Adamu said that between January and April this year at least 685 people had been kidnapped across the country.