The car in which they were travelling was overtaken on a bridge and forced to a halt. Gunmen opened fire and continued firing until all five occupants of the car, including the driver, were dead.
No group has claimed responsibility for the attack, though there is speculation that the killings may have been in retaliation for the conviction the previous day in Washington DC of a 33-year-old Pakistani immigrant, Aimal Kansi, for the murder of two CIA employees in January 1993.
Following the killings of the two men, Kansi, the sole suspect, fled the country and eluded capture for four years. He was eventually tracked down to a hotel in a remote part of the country in June. FBI agents were involved in his capture and he was then extradited to the United States without formal proceedings.
America is highly unpopular in Pakistan and is blamed for many of the country's ills. Aimal Kansi is seen as something of a hero - and if he gets the death penalty he will become a national martyr.
There may be a different explanation for the killings. Karachi is a violent city. Rival ethnic groups stage frequent shoot-outs, and more than 400 people have died in the city this year. One theory has it that the oil men's murderers are frustrated extortionists.
- Peter Popham, New DelhiReuse content