Ten people were injured in the explosion, including eight policemen and two civilians.
Al-Zarqawi's group claimed responsibility for the blast in a statement posted on the Internet, although the authenticity of the statement could not be confirmed.
There has been a string of car-bomb attacks by militants ahead of the 30 January national elections in Iraq. In an audiotape aired yesterday and attributed to al-Zarqawi, who is a Sunni, called for a war on the elections, which he said are a U.S. ploy to install Iraq's Shi'ites in power.
The checkpoint was the site of a similar attack in which three people were killed on 3 January.
Al-Zarqawi Lieutenants Captured
In the wake of today's attack, the Iraqi government announced in a statement that two of al-Zarqawi's top lieutenants have been captured.
The statement said Iraqi security forces captured Sami Mohammad Said al-Jaf, also known as Abu Omar al-Kurdi.
It said al-Kurdi was responsible for 32 attacks, including the August 2003 bombing of UN headquarters in Baghdad that killed more than 20 people, including the UN representative for Iraq Sergio Viera de Mello.
The second man was identified in the statement as Hamad Mohsen al-Duleimi, who allegedly was in charge of propaganda for al-Zarqawi's group.
Meanwhile, the U.S. ambassador to Iraq said yesterday that U.S.-led troops and Iraqi forces have "elaborate security plans" to defend the elections from militant attacks. Speaking on U.S. television, Ambassador John Negroponte, however, acknowledged there are some "problematic areas" where voting may be difficult.
"I would expect that we will see strong participation by Iraqi voters in the northern and southern parts of this country," Negroponte said. "There will be some problematic areas, particularly in the center, in the Sunni Triangle, especially the provinces of Al-Anbar and Nineveh."
For news, background, and analysis on Iraq's historic 30 January elections, see RFE/RL's webpage "Iraq Votes 2005".