BAGHDAD - An Iraqi legislator loyal to anti-U.S. Shiite cleric Muqtada al-Sadr was killed Thursday when a bomb struck his convoy in Baghdad, officials said.

The explosion hit the convoy carrying Saleh al-Auqaeili and other Shiite legislators from al-Sadr's parliament bloc as it passed about 200 yards from an Iraqi army checkpoint in a heavily secured area near Baghdad's main Shiite district of Sadr City, according to one of his colleagues.

Al-Auqaeili was taken to a hospital in Baghdad, where he later died of his wounds, according to Sadrist spokesman Ahmed al-Massoudi and a fellow lawmaker, Hassan al-Rubaie. At least one bystander was also killed in the bombing.

The U.S. military blamed Shiite extremists for the blast, which occurred shortly before 10 a.m. The attack raised concerns that internal rivalries within political groupings, whether Shiite or Sunni, could emerge ahead of upcoming provincial elections. Internal battles could be a major threat to Iraq's stability, even as sectarian violence ebbs.

Al-Auqaeili was a senior member of al-Sadr's political bloc, which has 30 seats in the 275-member parliament. The Shiite cleric's cease-fire order to his militia has been a key factor in a sharp decline in violence over the past year.

Al-Auqaeili was known for his moderate stance in the al-Sadr movement, where some factions are pushing for a return to violence, said another Sadrist legislator, who spoke on condition of anonymity to discuss sensitive internal issues.

Several followers of al-Sadr have been targeted in past attacks, but Thursday's bombing was notable because it occurred within a section of Sadr City that is considered secured by the U.S. military.

Sadr City and surrounding areas were long dominated by al-Sadr's Mahdi Army militia and were the site of fierce clashes between U.S.-Iraqi forces and Shiite extremists earlier this year. The area has been relatively peaceful after al-Sadr declared his ceasefire and the Iraqi security forces assumed control in late May.

Falah Hassan Shanshal, who was riding in a different car in the same convoy, said the group became suspicious when discovering there were no traffic jams in the usually crowded area. He said the "operation was a pre-planned one" and that the explosion was remote-controlled.

Shanshal also demanded a government investigation into how the roadside bomb was planted so close to the checkpoint. "We hold the security forces responsible for this attack. They should be responsible for the security of the city."

Iraqi police initially said the blast was caused by a motorcycle bomb. But Shanshal said the explosives had been planted in a hole near the road. He said a man on a motorcycle was killed, causing the confusion over the cause of the blast.

An Interior Ministry official, who declined to be identified because he wasn't authorized to release the information, confirmed it was a roadside bomb and said earlier that two other people were killed in the blast.

Also earlier, Maj. Mark Cheadle, a spokesman for U.S. forces in Baghdad, said the blast killed a civilian riding by on a motorcycle and wounded three other people in a car.

Tensions have been rising over the U.S.-Iraqi security talks on a deal that would replace the UN mandate for American forces, which expires at the end of the year.

Al-Sadr, who is believed to live in Iran but retains significant political clout in Iraq, strongly opposes the U.S. military presence. He and other critics fear that the new pact will bind the U.S. and Iraq in a long-term relationship, instead of restoring Iraqi sovereignty.

Al-Massoudi, the Sadrist spokesman, accused the Americans of targeting those who opposed the deal. "The occupation forces sent us a message by staging this attack because of our stance against the agreement," he said.