BAGHDAD - A passenger van packed with explosives blew up Friday at a bus station north of Baghdad where Shiite pilgrims had stopped for the night, killing four people and wounding dozens, U.S. and Iraqi officials said.

The blast happened a day after a female suicide bomber struck Shiite pilgrims traveling to Karbala for a major religious festival, killing at least 18 people and wounding 75.

Those attacks raised concern that extremists were seeking to re-ignite the firestorm of sectarian massacres that pushed Iraq to the brink of civil war two years ago before thousands of U.S. reinforcements were rushed to the country.

Hundreds of thousands of Shiites from throughout Iraq have been traveling by foot or by vehicle to Karbala, 50 miles south of Baghdad, for the religious festival.

U.S. and Iraqi officials said the blast occurred Friday evening at a bus terminal in Balad, a mostly Shiite town surrounded by Sunni villages about 50 miles north of the capital near one of the major U.S. military bases.

The U.S. military said three people were killed and 48 wounded. The director of the Balad hospital, Qassim Hatam, said four people had died and 40 were injured.

Balad has been relatively free from major attacks since May, and the U.S. military said nearly 600 former insurgents in the area had agreed to stop fighting and cooperate with the U.S. and its Iraqi partners.

Earlier Friday, a roadside bomb struck a minibus beginning the pilgrimage from Baghdad to Karbala, killing at least one passenger and wounding 10 others.

No group has claimed responsibility for the pilgrim attacks. Attacks on Shiite civilians -- especially during Shiite religious festivals -- have been the hallmark of Sunni extremists, including al-Qaida in Iraq.

The Shiite festival, Shabaniyah, celebrates the birth of Mohammed al-Mahdi, the 12th Shiite imam, who disappeared in the ninth century. Devout Shiites call him the Hidden Imam and believe he will return to restore peace and harmony.

The ceremonies reach their high point tonight and Sunday morning.

Security in Karbala

In Karbala, security troops searched pilgrims at the entrance to the city, seizing mobile phones and posters of Shiite religious leaders that might provoke attacks by followers of rival clerics. Mobile phones can be used to trigger bombs.

Police cars and ambulances roamed the streets, along with tanker trucks providing drinking water for pilgrims. City officials set up dozens of tents around the city to provide food, water and emergency medical care.

Dozens of army and police riflemen could be seen on buildings throughout the city looking for signs of trouble.

Last year, Shabaniyah celebrations were tarnished when armed clashes broke out between followers of anti-American cleric Muqtada al-Sadr and security forces controlled by rival Shiite groups.

Soon afterward, al-Sadr declared a cease-fire, in large part because of the backlash provoked among Shiites by his followers' role in the Shabaniyah clashes.

U.S. and Iraqi authorities are eager to prevent any flare-up of sectarian violence during the Shabaniyah ceremonies that might threaten the security gains of the past year.

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