BAIJI, Iraq -- The Baiji refinery may be the most important industrial site in the Sunni Arab-dominated regions of Iraq. On a good day, 500 tanker trucks will leave the refinery filled with fuel with a street value of $10 million.

The sea of oil under Iraq is supposed to rebuild the nation and then make it prosper. But at least one-third, and possibly much more, of the fuel from Iraq's largest refinery is diverted to the black market, according to U.S. military officials.

Tankers are hijacked, drivers are bribed, papers are forged and meters are manipulated - and some of the earnings go to insurgents who are still killing more than 100 Iraqis a week.

"It's the money pit of the insurgency," said Capt. Joe Da Silva, who commands several platoons stationed at the refinery.

Five years after the war in Iraq began, the insurgency remains a lethal force. The steady flow of cash is one reason, even as the U.S. troop buildup and the recruitment of former insurgents to American-backed militias have helped push the number of attacks down to 2005 levels.

In fact, money, far more than jihadist ideology, is a crucial motivation for a majority of Sunni insurgents, according to U.S. officers in some Sunni provinces and other military officials in Iraq who have reviewed detainee surveys and other intelligence on the insurgency.

Although many U.S. military officials and politicians - and even the Iraqi public - use the term al-Qaida as a synonym for the insurgency, some American and Iraqi experts say they believe that the number of committed religious ideologues remains small. They say that insurgent groups raise and spend money autonomously for the most part, with little centralized coordination or direction.

Money from swindles in Iraq and from foreign patrons in places like Saudi Arabia allows a disparate, decentralized collection of insurgent cells to hire recruits and pay for large-scale attacks.

But the focus on money is the insurgency's weakness as well as its strength, and one reason why loyalties can be traded. For now, at least 91,000 Iraqis, many of them former enemies of the U.S. forces, receive a regular American-paid salary for serving in neighborhood militias.

"It has a great deal more to do with the economy than with ideology," said one senior U.S. military official, who said that studies of detainees in American custody found that about three-quarters were not committed to the jihadist ideology. "The vast majority have nothing to do with the caliphate and the central ideology of al-Qaida."

A military official familiar with studies on the insurgency estimated that half of the insurgency's money came from outside Iraq, mainly from people in Saudi Arabia, a flow that does not appear to have decreased in recent years.

Before the invasion of Iraq, eight gasoline stations dotted the region around Sharqat, north of the refinery at the northern edge of Saddam Hussein's home province, Salahuddin. Now there are more than 50.

Gas stations are often built just to gain the rights to fuel shipments, at subsidized government rates, that can be resold onto the black market at higher prices. New stations cost more than $100,000 to build, but black market profits from six or seven trucks can often cover that cost, said officials who have studied the scheme.

The plan also requires bribing officials in the province and Baghdad, said Col. Mohsen Awad Habib, who is from Sharqat and is now police chief in Siniya, near Baiji.

In Baiji, dozens of active insurgent groups feed off corruption from the refinery, said Lt. Ali Shakir, the commander of the paramilitary Iraqi police unit here.

U.S. and Iraqi officials struggle to say exactly how much the insurgency reaps from its domestic financing activities. In the past, Iraqi officials have estimated that insurgents receive as much as half of all profits attributable to oil smuggling. And before the troop buildup began a year ago, a U.S. report estimated that insurgents generated as much as $200 million a year.

Nor is the skimming limited to the insurgency; illicit earnings from the Baiji refinery also flow to criminal gangs, tribes, the Iraqi police, local council members and provincial officials who also smuggle fuel, Iraqi officials say.

Barham Salih, the Iraqi deputy prime minister, said he believed that the pool of money available to insurgents across Iraq had fallen in the past year, but he declined to provide an estimate. He said Iraqi security analysts estimated that al-Qaida in Mesopotamia received $50,000 to $100,000 per day from swindles related to the Baiji refinery.

Those amounts are significant, given the hard realities of Iraq. Men can be hired to hide roadside bombs for $100, officers say.

The insurgents appear to understand how valuable the Baiji refinery is to their operations. "They have not attacked the oil refinery, because they don't want to damage their cash cow," said 1st Lt. Trent Teague, who commands the 3rd Platoon in Da Silva's unit, the headquarters company of the 1st Battalion, 327th Infantry.

Instead, when the insurgents want to send an angry message to someone at the refinery, they attack neighborhoods where oil workers live.