Recently discovered rockets ready to be fired exploded before they could be defused on Tuesday, killing at least 15 policemen and wounding 27, according to police and hospital officials who spoke on condition of anonymity because they were not authorized to release the information.

The blast occurred after police, acting on a tip, discovered the rockets primed for attack in the back of a truck behind a deserted ice factory in a predominantly Shiite area in eastern Baghdad. Explosives experts were trying to defuse the rockets, but two of them blew up in quick succession.

Elsewhere in Baghdad, the trial of two former Iraqi officials accused of letting Shiite militiamen use ambulances and hospitals to kidnap and kill rivals was delayed Tuesday because prosecution witnesses failed to show up, judicial officials said.

The two former Iraqi officials on trial, former Deputy Health Minister Hakim al-Zamili and Brig. Gen. Hameed al-Shimmari, are accused of having ties to the Mahdi Army, a militia loyal to anti-U.S. cleric Muqtada al-Sadr. Defense lawyer Amir Taha said the trial was put off until March 2.

The case is seen as a test of Prime Minister Nouri al-Maliki's willingness to crack down on Shiite extremists as well as Sunni insurgents. He is a Shiite who won office partly because of the support of al-Sadr's followers, though he has since fallen out of favor among them.

The delay avoids angering al-Sadr before he makes a decision on whether to renew a cease-fire he called that expires at the end of this month.

The cleric ordered his fighters in August to stand down for six months. The U.S. military has credited the cease-fire for a sharp drop in execution-style killings and kidnappings that had been attributed to the group. There is concern convicting the officials could create a backlash from Sadrists.

"If witnesses do not come the second time, it is possible that the court will depend on witness testimonies available in investigation documents," said Abdul-Sattar Bayrkdar, a spokesman for the Iraqi Higher Judicial Council.

Bayrkdar did not give a reason the witnesses did not come to court, but said no law exists to ensure their protection. He also said the new trial date was chosen in deference to Shiite festivities surrounding an upcoming holy day.

In the holy city of Karbala, the center of commemorations honoring the Shiite saint Imam Hussein, Iraqi police said they had arrested 56 militants over the past two days.

They included eight Sunni Al Qaeda operatives and 27 people who belonged to a Shiite cult, said Maj. Gen. Raid Shakir Jawdat, the provincial police chief. Police seized handguns, rifles, mortars and rocket-propelled grenades.

Jawdat said tens of thousands of police officers and soldiers would be deployed to provide security for pilgrims.

As the trial was postponed, Baghdad was again hit with an attack Tuesday in a reminder that, although safer than a year ago, the capital is far from secure.

A roadside bomb killed a taxi driver, a female passenger and her two young children, said a police officer, who spoke on condition of anonymity because he wasn't authorized to release the information. The blast occurred in the Sunni-dominated northern neighborhood of Waziriyah.

Also on Tuesday, unidentified gunmen in troubled Diyala province killed four people in a village just south of Baqouba where residents have turned against Al Qaeda in Iraq.

A 60-year-old man, his 45-year-old wife and their teenage son were slain at their home, along with a female relative, said an official in the joint police-military operations center, who also spoke on condition of anonymity because he wasn't authorized to release information.

The former Health Ministry officials' trial is being held at a new high-security compound in eastern Baghdad. Foreign security personnel were deployed around the building Tuesday.

Al-Zamili and al-Shimmari, who ran security for the ministry, are expected to face numerous charges, possibly including murder and kidnapping. U.S. soldiers stormed the officials' offices in separate raids last February.

After al-Zamili's arrest, the U.S. military said — without mentioning his name — that he was believed to have siphoned millions of dollars from the ministry to the Mahdi Army "to support sectarian attacks and violence targeting Iraqi citizens."

Militiamen also were allowed to use government hospitals and clinics to gather information on Iraqis seeking treatment and "those Iraqis that were discovered to be Sunnis would later be targeted for attacks," the military said.

Taha, the defense attorney, complained Tuesday that he was deprived of the right to meet with his clients.

"They did not allow me to meet them until now," he said. "I submitted five letters to meet them but they were postponed on the pretext of technical reasons." The defense team has said the allegations are baseless and plans to call more than a dozen witnesses.

Abu Firas al-Mutairi, a second defense lawyer and member of the Sadrist movement, said the trial would be a "political one taking the cover of criminal case."

"I hope the trial will not be affected by outside pressure," he said.