BAGHDAD, Iraq, (AP) – A suicide car bomber struck a busy commercial district Monday, killing at least 26 people and injuring more than 50, police said.

The attack near the well-known Mutanabi book market in central Baghdad was the first major blast in the city in several days — sending a huge pillar of black smoke as flames spread to shops, cars and book stalls.

The death toll from police was preliminary and could rise. At least 54 people were injured. The commercial zone is mixed between Sunni- and Shiite-owned businesses and shoppers.

“Papers from the book market were floating through the air like leaflets dropped from a plane,” said Naeem al-Daraji, a Health Ministry worker who was driving about 200 yards from the blast and was slightly injured by broken glass from his car window.

“Pieces of flesh and the remains of books were scattered everywhere,” he said.

People began driving the injured to hospitals in private cars without waiting for ambulances to arrive.

Sporadic car bombs and attacks have hit the capital in recent days, but none with mass casualties since a blast Friday killed at least 10 people.

In other violence, gunmen opened fire on Shiite pilgrims in two separate incidents around Baghdad on Monday, killing five people, police said.

On Sunday, U.S. and Iraqi troops poured into Baghdad’s main Shiite militia stronghold, encountering no resistance in the one-time Sadr City combat zones but testing the Shiites’ commitment to the U.S.-promoted campaign to drive militants from the capital.

Iraq’s prime minister, meanwhile, ordered an investigation into a British-Iraqi raid on a police intelligence headquarters in southern Iraq that captured an alleged death squad leader and found 30 prisoners with signs of torture.

West of Baghdad, U.S. soldiers described a raid last week that uncovered a suspected Sunni “torture site” and rescued of two Iraqi captives, who apparently had been spared immediate execution because the militants’ video camera broke and they wanted to film the killing.

The quiet but dramatic advance in Sadr City — involving nearly 1,200 U.S. and Iraqi forces who didn’t fire a shot — marked one of the most significant developments in the security clampdown in Baghdad since it took effect nearly three weeks ago.

But it only received the green light after drawn-out talks between U.S. commanders and political allies of radical Shiite cleric Muqtada al-Sadr and his powerful Mahdi Army. Both sides are watching each other for any wrong moves on the same streets where they battled in the past, including intense urban warfare in 2004.

Al-Sadr’s militiamen lowered their profile under intense government pressure to give the security operation a chance to root out both Sunni and Shiite extremists. U.S. military leaders, however, must walk a fine line as part of the tacit truce. They are seeking suspected Shiite death squads leaders, but must keep from squeezing al-Sadr’s militia too hard — and risk collapsing the entire drive to reclaim Baghdad from extremists and gangs.

“The indication that we are getting is a lot of the really bad folks have gone into hiding,” said Lt. Col. David Oclander shortly after troops moved into Sadr City’s teeming grid of low-rise buildings in northeast Baghdad.

Oclander said “not a shot was fired” as troops entered the area, which was constructed in the 1960s to house poor Shiites seeking work in the capital and was known as Saddam City until the former Iraqi leader’s fall in 2003.

As the insurgency picked up steam in the past few years, Sadr City became the site of frequent battles. Among the U.S. casualties was Spc. Casey Sheehan, whose death on April 4, 2004, began the anti-war campaign of his mother, Cindy Sheehan.

Last week, U.S. and Iraqi forces began pinpoint raids into Sadr City seeking suspected leaders of Shiite death squads blamed for thousands of execution-style slayings of Sunni rivals in recent years. Since Friday, military planners have worked inside a Sadr City police station in apparent preparations to create a permanent outpost, police said.

Despite the calm crossing into Sadr City, some quickly protested the strong U.S. presence.

An al-Sadr ally, lawmaker Falah Hassan, claimed the Sadr City pact called for Iraqi forces to lead the searches and only call in U.S. units if they faced resistance. He called the front-line U.S. role a “provocative act.”

Al-Sadr, too, has complained about the heavy U.S. role in the raids around the city. In a statement last week, he also decried the security plan’s inability to stop car bombs and other attacks blamed on Sunni insurgent groups against Shiite civilians.

The comments raised worries that he could order his Mahdi Army to confront forces carrying out the security operation. But he didn’t attempt to raise the stakes — a possible sign of newfound caution from al-Sadr.

Late last year, Washington strong-armed Shiite Prime Minister Nouri al-Maliki into pulling his protection for the firebrand cleric, who is based in the Shiite holy city of Najaf in southern Iraq and is a major political ally of the prime minister.

“We don’t know if he has a change of heart, but certainly there is a change of tactic,” the U.S. ambassador to Iraq, Zalmay Khalilzad, told CNN.

For a third consecutive day, Baghdad was spared a car bombing or attack bringing mass casualties. But there was still bloodshed. A Shiite newspaper editor and a police officer were gunned down.

After nightfall, U.S. artillery was heard across Baghdad. In recent days, U.S. gunners have hit suspected Sunni insurgent staging grounds south of the city.

Around the capital, U.S. forces reported the discovery of a series of weapons caches.

In southern Iraq, a joint British-Iraqi raid Sunday on the National Iraqi Intelligence Agency in Basra, about 340 miles southeast of Baghdad, discovered 30 prisoners, including one woman and two children, with signs of torture and abuse, the British military said.

The raid also captured an alleged death squad leader and four other suspected militiamen blamed for “kidnap, torture, murder and involvement in roadside bomb attacks on multinational forces and civilians,” said Maj. David Gell, a British military spokesman.

Al-Maliki, the prime minister, ordered an investigation into the raid and vowed to punish “those who carried out this illegal and irresponsible act.”

West of Baghdad, troops taking part in an operation that began Wednesday also stumbled upon an apparent hide-out west of Baghdad that was used by Sunni insurgents for torture and summary executions.

Lt. Col. Valery Keaveny described breaking through a double-locked door to find an Iraqi police officer and another Iraqi man who had undergone “considerable torture.” The policeman had been shot in both ankles and the other man had been dangling from the ceiling and “beaten severely by a pipe for a good deal of time,” Keaveny told reporters.

The captives told U.S. soldiers they had been convicted to death by an insurgent court at the site — about 18 miles west of Baghdad near the village of Karmah — and had the choice of either beheading or a fatal gunshot, said Keaveny.

They were spared immediate death, Keaveny said, because the insurgents’ video camera didn’t work and they had gone to get a new one to film the executions. “(The insurgents) said they would be back in the morning,” he said. “And that’s when we came in, that night.”

The two men were taken by U.S. forces for medical treatment.

The site also contained a huge stockpile of more than 1 million pounds of aluminum sulfate, which can be used as a component in nitrate-based fertilizer explosives. But it also has other commercial uses, including water purification. Aluminum sulfate was among the items found in the car of the so-called millennium bomber, Ahmed Ressam, an Algerian convicted in 2001 of plotting to bomb Los Angeles International Airport.

Elsewhere, the military said an airstrike Saturday in Arab Jabour, on the southern outskirts of Baghdad, forced insurgents to flee and leave behind four Iraqi hostages — one claiming he had been held for 50 days.

On Saturday, an al-Qaida-affiliated group posted a video purporting to show the execution of 18 kidnapped government security forces. The Islamic State of Iraq said it carried out the killings — with the victims being shot in the back of the head — in retaliation for the alleged rape of a Sunni woman by members of the Shiite-dominated police last month. The authenticity of the three-minute video could not be immediately verified.

The group also said it had killed 14 policemen, whose bodies were found Friday in the northeast province of Diyala. Some of the victims were decapitated, according to an AP photographer.

“It would be very helpful to the people of Iraq if civic and religious leaders, Sunni and Shia alike, would publicly denounce these horrific acts,” said Maj. Gen. Benjamin R. Mixon, commander of U.S. forces north of Baghdad. “With many influential leaders making the same statement, the population will realize that these terrorists are not the future of Iraq.”