The blast hit Lieutenant Colonel Giuma Misrati just after he left his home and was standing next to the vehicle, the authorities added. Libya remains anarchic and awash with weapons nearly two years after a Western-backed uprising toppled dictator Muammar Gaddafi.
Government forces are still struggling to assert their authority on the major oil producer, but have been regularly challenged by regional militias who want to keep hold of the influence they gained during the revolution.
Benghazi, the cradle of the 2011 revolt, has became a particular focus of violence, mainly against security forces. Residents say some of the assaults may be revenge attacks by former prisoners. Islamists, largely suppressed under Gaddafi, have also come to the fore.
"There are some people who are trying to use this city as an arena for settling scores," said the deputy head of Benghazi's local council, Saad al-Saity.
"This is impeding our plans for reconstruction and getting foreign companies to return," he added. On Tuesday six soldiers were killed in an attack by unidentified gunmen on an army checkpoint south of the coastal city of Sirte.
Last week, bombs flattened a police station in Benghazi but there were no casualties as the building had been emptied for repairs following previous attacks.
The city remains a no-go area for foreigners, after a series of attacks on Western diplomats including last September's assault on the US mission which killed the ambassador and three other Americans.