In the past three months, Iraq has witnessed some of the worst violence since the fall of dictator Saddam Hussein in April, 2003. Car bombings have killed nearly 600 people and wounded 1,700 more. Some U.S. and Iraqi officials believe Iraqi Sunni extremists may be joining foreign fighters in adopting suicide car bombings as their primary weapon in the insurgency.

According to U.S. military statistics, Iraq suffered through a staggering number of car bombings in the past three months.

A report shown to reporters indicates that at least 232 suicide car bombings, some remotely detonated, occurred between April and June, and that number does not include the nearly 50 car bombs that were discovered and defused.

U.S. officials say they believe the majority of suicide bombings are being carried out by foreign fighters infiltrating Iraq's porous borders. But some intelligence analysts are expressing alarm about finding Iraqi extremists behind some of the recent car bombings.

Last month, media reports quoted senior American military officials who said that Iraq's most wanted terrorist, Abu Musab al-Zarqawi, met several times with the leaders of various Iraqi insurgent groups in neighboring Syria and western Iraq sometime before April.

The officials said that a flurry of suicide car bombings followed the meetings, suggesting that Iraqi insurgent factions, including al-Qaida-inspired militants, former Saddam loyalists, Sunni Arab radicals, and common criminals, may have agreed to shift strategy and coordinate their efforts to achieve more deadly results.

A spokesman for the multi-national forces in Iraq, Brigadier General Donald Alston, notes that between April and June, other types of attacks, especially against infrastructure such as power stations and pipelines, declined sharply.

"They have gone to more spectacular systems that can inflict more casualties per attack, likely because they cannot sustain high-volume attacks," he said. "So, that shift to the car bomb is certainly a distinctive shift. And he does not have to have success 100 percent of the time. If he fails four out of five times, but the one time is in a market place where a hundred people are killed, he has achieved a great deal of what he is trying to achieve."

General Alston and Iraqi leaders say recent military sweeps in Baghdad and western Anbar province have led to the arrests of numerous insurgents, including at least 20 of Abu Musab al-Zarqawi's top lieutenants.

Tips from residents have led to the discovery of sophisticated car bomb factories in the Iraqi capital, where insurgents could rig a car with explosives in less than an hour.

But General Alston acknowledges that suicide car bombings are likely to remain a favorite weapon of insurgents determined to frighten the Iraqi people into submission and undermine progress toward democracy.

"The enemy gets to pick the time and the place in order to achieve the effect that they are trying to achieve," he said. "That is a challenging problem to solve. Our ability to adapt and challenge him with the Iraqi security forces has continued to improve and we are seeing some success. But I think the problem will continue in Iraq for a period of time."

The commanding general of U.S. forces in the Middle East, General John Abizaid, says that he cannot confirm or deny that meetings between Abu Musab al-Zarqawi and Iraqi insurgent leaders took place in Syria. But he has urged the Syrian government to do more to secure its borders to keep violence from migrating into Iraq.