Police and Shiite Muslim protesters clashed in a northern city Thursday, killing one man, while investigators named an Al Qaeda-linked militant group as their chief suspect in the suicide bombing of a Shiite mosque in Karachi (search) that triggered mass rioting.

Dozens of minority Shiites ransacked a state-run radio station and attacked other government offices in the northern city of Gilgit (search), during a protest against textbooks they say go against their faith.

Authorities had imposed a round-the-clock curfew to prevent the protest, fearing that tensions from Shiite-Sunni violence in the southern port city of Karachi this week would spill over into Gilgit.

The protesters, defying the order, pelted security forces with stones and opened fire, wounding two paramilitary soldiers and two policemen, a government official said on condition of anonymity.

Security forces returned fire, killing a youth and wounding two other protesters.

Police arrested eight people, including Shiite leader Agha Ziaudin.

Violence in Karachi, Pakistan's largest city with 14 million people, erupted Sunday after gunmen assassinated a prominent Sunni Muslim cleric. A day later, a suicide bomber attacked the Shiite mosque, killing 21 people.

The attacks brought mobs of Shiite (search) and Sunni (search) youths to the streets, and some officials suspect the attacks were aimed at igniting sectarian unrest.

Senior police investigator Gul Hameed Sammu said the suicide bombing was similar to an attack May 7 at a Shiite mosque in Karachi that killed 22.

Police suspect that Lashkar-e-Jhangvi, a radical Sunni extremist group with members linked to Al Qaeda and the murder of American journalist Daniel Pearl, was behind both bombings, he said.

"We found similarities," Sammu said. "Both were suicide bombings, but in latest one the culprits used a little extra explosive that caused more severe damage."

In Washington, Richard Boucher, the U.S. State Department spokesman, declined to speculate if Al Qaeda was involved, but told reporters: "There are local organizations and leaders that have been promoting the violence or the demonstrations."

About 80 percent of Pakistan's 150 million people are Sunni Muslims and about 17 percent Shiites. Karachi has long been a flashpoint for Pakistan's religious and ethnic divisions.

Meanwhile, Pakistani security forces killed a foreigner who hurled a hand grenade at a checkpoint in a tribal region near the Afghan border, injuring two paramilitary soldiers, an army spokesman said.

Security forces nearby arrested four other foreign men traveling in a vehicle from Afghanistan with grenades and other weapons inside. Three of the men were concealing themselves in burqas, the traditional, all-covering women's dress.

Maj. Gen. Shaukat Sultan described the man who threw the grenade as a "suspected terrorist." He refused to disclose the nationalities of any of the foreigners.

Hundreds of Central Asian, Arab and Afghan militants with suspected Al Qaeda links are believed to be sheltering near the Afghan frontier. The region is thought to be a hiding place for militants launching attacks against U.S. and Afghan forces in eastern Afghanistan.