Police officials said two armed militants riding on a motorcycle opened fire at the vehicle of Raza Khan, former police assistant sub inspector in Kharotabad area of Quetta.
They said Khan died on the spot and the attackers drove off after the attack last evening.
Khan was an accused in the murder of five Chechen nationals, including two women, in Kharotabad.
Security forces and police killed the Chechen nationals on May 2011 for allegedly possessing suicide jackets and other arms and ammunition.
After media reports, an inquiry was conducted by the Balochistan government and nothing was found with the dead Chechen nationals.
On December 29, 2011, police surgeon and a key witness in the incident Dr Baqir Shah was shot dead by unknown gunmen in Quetta.