Provincial Governor Adnan al-Zurfi said at least one suspect was an Arab who held a foreign passport.
The attack followed a similar blast in nearby Karbala. Yesterday's total toll in the two Shi'ite holy cities was at least 62 dead and more than 180 wounded.
There has been no claim of responsibility for the apparently coordinated car bombings on 19 December, which left 48 dead in Al-Najaf and 14 dead in Karbala.Shi'ite Leaders Call For Restraint
After the bombings, Shi'ite Muslim leaders urged their supporters to show restraint.
Grand Ayatollah Mohamed Said al-Hakim warned attackers were "trying to spark sedition and destabilize the country."
Cleric Muhammad Bahr al-Uloum said attempts to ignite a sectarian war in Iraq and prevent the 30 January elections from going forward as scheduled had failed before and would fail again.
The attacks were the first major violence in the two holy cities since clashes earlier this year between U.S.-led forces and Shi'ite militiamen.
In Baghdad yesterday, gunmen pulled three Iraqi electoral officials from a car and executed them in a street of the capital.
(Reuters/AP/AFP)For more on events in Iraq, see RFE/RL's dedicated The New Iraq webpage.