Iraqis vented anger Monday at a major security lapse that allowed two suicide truck bombers to penetrate what was supposed to be one of Baghdad's safest areas, killing 155 people, including 24 children leaving a daycare center.

Sunday's twin suicide blasts in the heart of the capital targeting the Justice Ministry and the Baghdad Provincial Administration were the worst attacks in more than two years. They raised fears about Iraq's ability to protect itself as it prepares for January elections and the U.S. military withdrawal.

The children who were killed were on a bus leaving a daycare center near the Justice Ministry when the attack occurred, said an official at the hospital where the bodies were brought. A police official confirmed the death toll and said the bus driver was also killed and six children injured. The officials spoke on condition of anonymity because they were not authorized to talk to the media.

Baghdad's top security officials brought reinforcements to guard government institutions Monday and blocked streets around the capital, acting on intelligence suggesting such buildings could be the militants' next target, Maj. Gen. Qassim al-Moussawi, a spokesman for the city's operations command center told The Associated Press.

But those measures seemed insufficient to many angry Iraqis who fear more attacks and question the ability of Prime Minister Nouri al-Maliki's government to keep the country secure.

"This explosion made people furious," said Ahmed Hassan, an employee at the Ministry of Education. "People will not re-elect this government."

Al-Maliki has staked his political reputation and re-election bid on his ability to bring peace to the country. But grief turned into anger Monday — an ominous sign for his re-election bid.

There have been no claims of responsibility, but massive car bombs have been the hallmark of Sunni insurgents seeking to overthrow the country's Shiite-dominated government. It was the second deadly bombing targeting government offices in the area since August, when coordinated blasts against two ministries killed more than 100 people.

The death toll rose to 155 on Monday as Baghdad residents buried the dead. About 500 people were injured, authorities said.

The initial investigation suggested the vehicles, each packed with thousands of pounds of explosives, might have passed through some security checkpoints before hitting their destination, al-Moussawi said.

Baghdad's top security officials reviewed the footage taken by security cameras in the area of Sunday's blasts, al-Moussawi said. The footage showed a 26-seat bus targeted the Justice Ministry, and a minibus, with up to 10 seats was detonated at the Provincial Administration.

The interior and the engine of the first vehicle were heavily modified to accommodate 2,205 pounds (1,000 kilograms) of explosives, al-Moussawi said, adding the minibus carried 1,543 pounds (700 kilograms) of explosives.

Both bombings were suicide attacks, al-Moussawi said. The remains of the attackers' bodies were recovered from the vehicles' wreckage, and investigators are trying to determine the bombers' identities, he added.

Interior Minister Jawad al-Bolani said 76 people have been arrested so far, but he did not provide information on who they were or how they are believed to be connected to the horrific crimes, which took place just hundreds of yards from the heavily fortified Green Zone.

"This is a terrorist act," al-Bolani said. He called on all the political forces to cooperate and assist the Iraqi security forces.

In Washington, Pentagon spokesman Bryan Whitman said the U.S. military is providing bomb disposal and forensic help to Iraqi authorities at their request.

The street where the blasts occurred had been reopened to vehicle traffic just six months ago. Shortly after, blast walls were repositioned to allow traffic closer to the government buildings — all measures hailed by al-Maliki as a sign that safety was returning to the city.

"Today, we came to work despite the fear inside us," said Siham Abdul-Karim, 49, an employee of the Culture Ministry located near the site of the bombings and surrounded by police checkpoints on Monday. "We all wonder how could car bombs reach these institutions."

Iraqi health and security officials confirmed the death toll. They spoke on condition of anonymity because they were not authorized to release information to the media.

The coordinated bombings were the deadliest since a series of massive truck bombs in northern Iraq killed nearly 500 villagers from the minority Yazidi sect in August 2007. In Baghdad itself, it was the worst attack since a series of suicide bombings against Shiite neighborhoods in April 2007 killed 183.

Also Monday, a car bomb at a police checkpoint near the holy city of Karbala in southern Iraq killed at least four people, Iraqi security officials said on condition of anonymity because they were not authorized to speak to the media.