Bombs pounded six Iraqi cities and towns on Tuesday, killing at least 40 people and raising suspicion that security forces might be assisting terrorists in launching attacks on Shiite Muslims.

The onslaught came just ahead of a religious pilgrimage that could attract even more violence.

A senior Iraqi intelligence official said checkpoint guards may have been bribed to help al-Qaida-linked Sunni insurgents plant bombs at Shiite marketplaces. The attacks injected new fear into Iraqis, resigned to worsening violence six months after the last American troops left the country.

"We want to live a normal life, but with the current spike in violence and victims, I am personally thinking of moving," said Hassan al-Saadi, 40, a Shiite sports equipment store owner in Baghdad who is considering pulling his four children from school for their safety.

"I see the future as worse," al-Saadi said.

A spike in violence over the last month is blamed partially on Iraq's paralyzing political crisis, which pits Prime Minister Nouri al-Maliki's Shiite-led government against rival Shiite politicians, Sunni Muslims and ethnic Kurds who complain they've been sidelined.

Also, the crisis in neighboring Syria may have allowed weapons intended for the opposition to President Bashar Assad to be siphoned off to Iraqi insurgents.

Tuesday's deadliest attacks hit the southern Shiite cities of Karbala and Diwaniyah. Despite the risk, hundreds of thousands of Shiite pilgrims are expected to gather Friday in Karbala for an annual religious observance.

Sunnis also were targeted. Two blasts hit a residential area in the Sunni city of Taji, just north of the capital, killing three people. Four people died in bombings and shootings in Sunni-dominated Diyala province in Iraq's northeast.

In Diwaniyah, officials said an explosives-laden vegetable truck ripped through a crowded market, killing 26 people and wounding about 75 more.

"There were many charred bodies on the ground," said vegetable seller Salah Abbas, 41, who rushed to help wounded people before ambulances arrived. "People screaming and crying - some were coming in to get their relatives while others were running out."

The senior intelligence official said there were at least two security lapses in the market attack, and money might have changed hands.

One guard at a security checkpoint in Diwaniyah failed to properly search the produce truck because he couldn't stand the smell of rotting vegetables and fruit. Another guard allowed the truck to enter the market instead of being unloaded outside as security rules require, the intelligence official said.

"We do not rule out that bribes were paid to some at the checkpoints," the official said, speaking on condition of anonymity in order to discuss sensitive security matters.

Checkpoints are so common in Iraqi cities that it's impossible to go more than a few kilometers (miles) without encountering one.

"The aim of today's attacks is to show that all the security measures taken to protect the pilgrims are a failure," the intelligence official said.

A few hours before the Diwaniyah attack, two bombs in cars parked outside a Karbala market killed five people and wounded 30. Karbala is 90 kilometers (55 miles) from Baghdad, and Diwaniyah is further south, 130 kilometers (80 miles) from the capital.

In Baghdad, two roadside bombs exploded near security patrols in separate neighborhoods, killing a policeman and a passer-by, officials said.

In Diyala province, just northeast of Baghdad, a bombing killed two farmers, and a drive-by shooting killed two security officers. The casualties in Baghdad, Taji and Diyala were confirmed by police and health officials who spoke on condition of anonymity because they were not authorized to release the information.

There were no claims of responsibility for the latest bombings, which followed a particularly bloody June.

Last month, attacks timed around a Shiite pilgrimage in Baghdad killed about 100 people. This week, pilgrims are heading to Karbala, one of the holiest cities in Shiite Islam, to mark the birth of the ninth-century Shiite leader known as the Hidden Imam.

John Drake, an Iraq specialist for the British-based AKE security consultancy, said attacks on pilgrims are likely to increase in coming days.

"A large number will travel to Karbala this week, and radical groups such as al-Qaeda and its local affiliates may try to attack them in an attempt to inflict numerous casualties and enflame inter-communal tensions," Drake said.

In the years after the 2003 U.S.-led invasion toppled Saddam Hussein's Sunni regime, Shiite leaders encouraged pilgrimages to demonstrate their power. Slain pilgrims were considered martyrs and strengthened the Shiite cause by demonizing Sunnis. Even now, with Shiites controlling the government, the pilgrimages continue in a show of faith largely banned under Saddam.

"We are confident that the Shiite pilgrims will be undaunted by these explosions," Karbala Gov. Amal-Din al-Hir said.

Security forces have struggled to stop past month's spiraling violence, damaging the government's shaky credibility among Iraqis and fanning fears the country may be spiraling out of control without American troops.

Sunni insurgents appear to have been emboldened by the months-long political crisis, seeking to exploit ethnic and sectarian tension to drag Iraq back down into the widespread violence between Sunni and Shiites that brought Iraq to the brink of civil war five years ago.

"These guys see Iraq as a gigantic bomb, and they are trying their hardest to set it off," said Kenneth Pollack, a Clinton administration official who is now a Middle East analyst at the Brookings Institution think tank in Washington.

Though there have not been retaliatory attacks by Shiite militias that characterized Iraq's darkest days, Pollack sees no quick relief for Iraq. He said security forces are too weak to stop terrorist attacks, and any harsh crackdown by al-Maliki's government could prompt even more resentment and unrest among Sunni and Kurd populations.

"Right now, the most likely trend is deepening violence," Pollack said.