Widespread, many experts say. Insurgents frequently appear to have inside information about the movements and routines of Iraqi and U.S. troops that they use to mount deadly attacks, such as the December 21 suicide bombing of a U.S. military base in Mosul, which killed 22. Though the Pentagon does not provide specific numbers, news reports indicate that dozens of Iraqi police and military personnel, including some high-ranking commanders, have been dismissed on suspicion they provided information or other assistance to insurgents. "Subversion of the government and armed forces is the bread and butter of an insurgency," Bruce Hoffman, a RAND Corporation counterinsurgency expert, recently told the Associated Press.

There are three broad categories, and individuals in each have different motivations, experts say:

• Hard-core fighters. These are insurgent fighters who pass limited background checks and are inducted into security forces or given jobs on U.S. bases. In some areas--such as towns in the Sunni triangle, the heart of the insurgency--up to one-fifth of recruits may be insurgents, says U.S. Army Special Forces Major James A. Gavrilis, an international affairs fellow at the Council on Foreign Relations who trained Iraqi security forces in 2003 and 2004. In less violence-prone areas where the population is cooperating with the political transition--such as Shiite areas in southern Iraq--this number is likely much lower, maybe only 1 percent, Gavrilis says.
• Sympathizers. These security personnel do not personally take up arms against Americans or the Iraqi government, but they may provide some information or assistance to insurgents. There are likely many more sympathizers than insurgents in Iraq's forces, experts say. They may be motivated by payments from the insurgency--which is reportedly financed by funds under the control of Saddam Hussein loyalists--hatred of the U.S. occupation, or tribal or ethnic ties. Many Iraqi police and soldiers return home each day after work, where they may come into contact with family members and neighbors linked to the insurgency who demand help, says retired three-star Marine General Bernard Trainor, an adjunct senior fellow at the Council on Foreign Relations and author of "The Generals' War: The Inside Story of the Persian Gulf War."
• Coerced and intimidated security forces. These police officers and soldiers may genuinely want to work with the new Iraqi government but share information with insurgents out of fear. More than 2,000 Iraqis serving in the security forces have been killed in insurgent attacks since May 2003.

Not with precision, experts say. Last October, Aqil al-Saffar, as aide to interim Prime Minister Ayad Allawi, said that as many as 5 percent of the Iraqi government's troops were insurgents or sympathizers, The New York Times reported. Some experts suggest the number may be higher. "Penetration of Iraqi security and military forces may be the rule, not the exception," Anthony Cordesman, a military analyst at the Center for Security International Studies, said in a January report. Some U.S. commanders agree. "The police and military forces all have insurgents in them. You don't have a pure force," Lieutenant Colonel Jeffrey Sinclair of the 1st Infantry Division told the Associated Press. However, focusing on the size of the infiltration misses the point, Gavrilis says. "I don't think the [overall] level of penetration is as much as most people think it is, because you don't really need a lot to do a lot of damage."

All recruits are vetted. It's the 'first line of defense,' Trainor says. But many experts say the vetting of new Iraqi forces has been inadequate and rushed. Many files on individuals from Saddam Hussein's regime are scattered, destroyed, or contain unverifiable or outdated information of limited use to current commanders. Iraqis lack sufficient personnel to conduct in-depth background checks on the thousands of new soldiers and police officers who join security forces each month. 'This is part of the downside of the fact that the most important goal has been to create as large a force as possible as quickly as possible,' says retired Army Colonel Andrew Bacevich, a professor of international affairs at Boston University.

Through a mixture of background checks, interviews, physical fitness tests, and observation of their performance on the job, experts say. Iraqi officers ask recruits basic questions about their loyalties, such as whether they support the Iraqi government. All recruits also have to provide at least one reference from someone in their town or village whom commanders judge trustworthy, General Babaker Shawkat Zebari, the chief of staff of the Iraqi army, told the Chicago Tribune. Iraqi military intelligence--which is being trained by U.S. intelligence services--then seeks to verify these references and examine the recruits' background. Finally, names are checked against CIA and other U.S. databases to make sure no one with a recent prison record--or is a so-called high-value target from Saddam Hussein's regime--can make it onto the force.

Since the handover of sovereignty to Iraqis in June 2004, Iraqi officials have become largely responsible for vetting their own forces, U.S. commanders say. "We don't do a systematic vetting process on Iraqi security forces, their government does that," Lieutenant Colonel Dan Wilson, deputy for current operations for the 1st Marine Division, told the Associated Press after the Mosul bombing. Even before last June, however, U.S. commanders often relied on trusted Iraqi counterparts to vet forces, because they were familiar with the country and better able to judge recruits' backgrounds. Checks for low-level police and other security jobs were at times quite cursory. In December 2003, The Washington Post reported that vetting of local police at one Baghdad station consisted of a two-minute interview in which an Iraqi commander "sized up" the candidates with a few questions about their backgrounds and political opinions.

No. It is generally believed to be considerably more comprehensive in Iraq's small, professional army--which now has some 4,100 men--than in the police, where turnover rates and recruit numbers are much higher. There are now 53,000 trained police, but the number has varied widely: in March 2004, the force had 70,000 men, and in October 2004, there were 84,950 officers on duty, according to U.S. totals. Iraqi officials say they are still conducting background checks on police officers hired before June 2004. "We have a lot of trouble getting information; we still have 50,000 [Iraqi police] who haven't been checked out," Interior Ministry spokesman Sabah Kadhim told reporters December 26. "And it's not just a question of loyalty or background; we don't even know whether they're working."

U.S. forces, with the help of the Iraqi military, officials say. Investigators screen truck drivers, mess hall workers, and other employees and check them against a U.S.-compiled database of criminals and former regime members, Army Captain Darren Luke, a U.S. military spokesman in Baghdad, told The Boston Globe. Iraqis are then issued identity cards, which they must show at least once when entering a base, and are subject to comprehensive searches for explosives and other materials. Still, U.S. military commanders say they believe their bases are infiltrated; insurgents carrying base ID cards are occasionally captured.

An investigation is ongoing, but it appears he was not, according to Iraqi officials and news reports of the U.S.-led probe's preliminary findings. He was wearing an Iraqi National Guard uniform, investigators agree, but could have bought it illegally at the market in Mosul, where they are readily available. The suicide bomber "was not a member of the national guards because all of our men stationed in the base have been accounted for," Zebari, the army chief of staff, told the Associated Press. On the other hand, the Islamist group that claimed responsibility for the bombing, Ansar al-Sunna, clearly had knowledge of the schedule on the base, indicating the bomber had some inside information.

Among them:

• The carefully planned roadside massacre of 50 unarmed Iraqi cadets headed home on leave October 23.
• The September arrest in Dyala province of a senior Iraqi National Guard commander, Lieutenant General Talib al-Lahibi, for links to the insurgency.
• The October arrest of Iraqi National Guard battalion commander Colonel Daham Abd, for allegedly providing ammunition, money, and information to Kirkuk insurgents.
• Numerous other attacks on Iraqi security forces, especially those that occur while large numbers of them are gathered for a ceremony or other events and are particularly vulnerable.

By slowing the hiring process and increasing the length of training of forces, many experts say. Observing the police and soldiers at work--and whether they are willing to risk their lives in combat situations--is perhaps the clearest way to test recruits' loyalty, Gavrilis says. Iraqis also need assistance in developing an effective counterintelligence agency within the police and military that can help root out disloyal forces, Trainor says. In addition, a professional Iraqi officer corps will help ensure loyalty. Building such a system from scratch can take years, experts say. There has been more emphasis placed on training and vetting since June 2004, when the military appointed Army Lieutenant General David H. Petraeus to head the Multinational Security Transition Command in Iraq, the unit in charge of training Iraqi security forces. But experts say procedures still appear to be inadequate. After the Mosul attacks, U.S. officials announced that a military assessment team led by Army Brigadier General Richard P. Formica would reassess security and vetting procedures, both on bases and within the Iraqi forces.

The most immediate effect is that it allows insurgents to get information about troop movements and plans. It also sows distrust between U.S. forces and Iraqi troops, Trainor says. And the presence of insurgent infiltrators undermines trust and cohesion—essential elements for any effective security force. "The U.S. military can do a great job training the Iraqis, but if there is this rot on the inside that is eating away at the attitude of the forces, I'm not sure training will be enough," Bacevich says. "It's an enormously difficult and potentially fatal problem."

Yes. As in Vietnam, where there was widespread Vietcong subversion of the U.S.-backed South Vietnamese army, some experts say the Iraqi infiltration may be part of a guerrilla strategy to seize power after the January 30 elections. "Many in the Iraqi security forces are simply biding their time right now; they are in league with the insurgents but are not showing their hand," says Kenneth Katzman, senior Middle East analyst at the Congressional Research Service. "There is a core hoping to get integrated into the post-election force with the intention of staging a coup d'etat." Many insurgents sympathize with Saddam Hussein's Baath party, Katzman says, "and the Baath Party strategy has always been to get control of the security forces." Some experts predict Iraqi Shiites, who make up 60 percent of the Iraqi population and who won a majority of transitional National Assembly seats in the January 30 elections, will purge the forces and replace many Sunni officers with Shiites. Others say that if the U.S. military remains engaged in Iraq, it will limit purges and attempt to ensure the Iraqi army remains an ethnically and religiously balanced force that won't tyrannize Iraqi minorities.