Deputy inspector-general Hassan Salim and three other employees were driving to their office in the northern oil city of Kirkuk when assailants in a speeding car sprayed their minibus with bullets.
Salim and another employee were killed instantly and the two other passengers were seriously wounded, police sources said.
Nobody claimed responsibility for the killings and police investigations have yet to uncover the perpetrator, according to a police officer in the investigating team
The number of bombings in the city has decreased since the Kurds took over, but there has been a wave of assassinations, many of them targeting Arabs.
Kirkuk is subject to competing claims of Kurds, Arabs and Turkmen and has long been a flashpoint.
NOC's chief of operations, Saad al-Karbalaie, was killed in late June. Over a month later, his successor at the state-run firm chief engineer Saad Ali Hussain was also gunned down.
Two NOC officials who requested anonymity told Reuters that the deputy inspector-general was conducting multiple corruption probes inside the company.
"When you have an official running a corruption investigation getting killed, then it's a bloody message from parties that could get exposed to stop chasing them," one of the officials said.
"Until now we have lost three officials and more to come. North Oil Company is becoming more of a mafia courtyard rather than a state-run firm."
(Reporting by Mustafa Mahmoud; Additional reporting by Ahmed Rasheed; Editing by Michael Georgy and Tom Heneghan)