BAGHDAD, Iraq — A suicide car bomber shattered a relative lull in Baghdad's violence today, killing at least 28 people in a blast that touched off raging fires and a blizzard of bloodstained paper from a popular book market.

It was the largest bombing in the capital in three days, and came on the heels of a major push by nearly 1,200 U.S. and Iraqi troops into Sadr City, a Shiite militia stronghold and base for fighters loyal to radical cleric Muqtada al-Sadr.

Iraqi troops in Sadr City set up checkpoints and took a far more visible presence than Americans, who led the push into the area Sunday. The move was an apparent attempt to avoid Shiite anger in a place of past street battles with U.S. forces.

But pressure on al-Sadr and his Mahdi Army militia continued on other fronts.

In the southern city of Karbala, the home of a Mahdi Army leader was raided in a joint U.S.-Iraqi operation, the U.S. military said.

Al-Sadr's followers also warned the Iraqi government they would not relinquish Cabinet posts unless other members of the ruling coalition do the same — setting the stage for a major political battle as Prime Minister Nouri al-Maliki prepares to reshuffle his administration.

Meanwhile, a British-Iraqi raid on a police intelligence headquarters in southern Iraq on Sunday found 30 prisoners with signs of torture and an alleged death squad leader was captured.

The raid took place at the National Iraqi Intelligence Agency building in Basra, Iraq's second-largest city 340 miles southeast of Baghdad. Inside, troops discovered the prisoners, including one woman and two children, with signs of torture and abuse, the British military said in a statement. It did not elaborate.

The Iraqi government on Monday launched an investigation.

"We have a committee that ... has started work to uncover the circumstances of the incident," said Ali al-Dabbagh, a government spokesman.

Al-Maliki had ordered the inquiry a day earlier, and vowed to punish "those who carried out this illegal and irresponsible act," his office said. It was not immediately clear whether the complaints concerned the raid itself or the apparent torture of the prisoners.

Today, black smoke drifted over central Baghdad from burning shops, cars and book stalls in the mixed Sunni-Shiite area around Mutanabi street along the Tigris River. At least 66 people were wounded in the suicide blast.