The bomb blew out a side wall of the building, leaving desks, filing cabinets and computers strewn among the concrete rubble. It also damaged the Benghazi branch of the Libyan Central Bank along a major thoroughfare in the city.
The Foreign Ministry used the building to provide government services to Libyans and foreigners in the eastern region, which is hundreds of kilometers (miles) away from the capital, Tripoli.
The explosion came a day after authorities found and defused another bomb next to the Foreign Ministry building in Tripoli, Libyan Prime Minister Ali Zidan said.
Speaking to journalists hours after the explosion, Zidan pledged the government would track down those responsible. "There is a force that wants no state and to turn Libya to a battlefield of terrorism and explosions," he said.
Deputy Interior Minister Sadik Abdel-Karim said the country's security situation was "deteriorating."
"The message has been delivered to every Libyan — especially in Benghazi," he said.
Libya has no interior minister since the last one resigned over a conflict with Zidan weeks ago.
Gadhafi was killed after an eight-month uprising that descended into a civil war in 2011. Since then, successive Libyan interim governments have failed to impose law and order. The country remains held hostage by unruly militia forces initially formed to fight Gadhafi. The militias, which have huge stockpiles of sophisticated weaponry, now threaten Libya's nascent democracy.
Zidan acknowledged the challenge Wednesday, saying that "the security situation is tough." Former Interior Minister Ashour Shwayl said that as long as the military and police are not in place, the turmoil will continue.
"To sum it up, there is no solution but for the police, military and judiciary are built up," Shwayl told The Associated Press in a telephone interview. "Chaos otherwise will remain."
Car bombs and drive-by shootings since the end the civil war also routinely kill security officials in Benghazi, the birthplace of the uprising.
"Even with so many officials assassinated, no one is held accountable," said Tawfiq Breik, a lawmaker with the liberal-leaning National Forces Alliance. "No one is arrested. The state is disabled."
The car bombing comes exactly one year after al-Qaida-linked militants stormed the U.S. mission in Benghazi and a nearby U.S. building, killing U.S. Ambassador Chris Stevens and three other Americans.
"We can't ignore the date and timing. We can't forget," Zidan said, suggesting that the car bombing was meant to be a reminder.
The attack sparked a wave of criticism toward President Barack Obama and his administration for its handling of the attack and its aftermath. The administration closed 19 diplomatic posts across the Muslim world for almost a week last month out of caution over a possible al-Qaida strike — likely in response to the Benghazi criticism.