PATROL BASE HAWKS, Iraq, (AP) – When U.S. sentries fatally shot three guards near an Iraqi-manned checkpoint south of Baghdad, they thought they were targeting enemy fighters planting roadside bombs, according to the American commander of the region.

The shootings, which are still under investigation, underscore a new dilemma facing U.S. troops as former fighters join forces against extremists and Iraqis are increasingly forced to take up arms to protect themselves — how does one distinguish them from the enemy?

The U.S. military said the American troops shot the three civilians Thursday near a checkpoint manned by local members of a U.S.-allied group helping provide security in the village of Abu Lukah, near Musayyib, a Shiite-dominated town 40 miles south of Baghdad.

Maj. Gen. Rick Lynch, commander of the 3rd Infantry Division that controls territory south of Baghdad, stressed the investigation was continuing but said initial results showed that U.S. troops fired on the checkpoint after spotting what appeared to be enemy forces planting roadside bombs.

“We are not looking to see who made a mistake but rather see what we can learn from that particular event,” Lynch told The Associated Press Saturday during a whirlwind tour of patrol bases in the area.

Lynch said it’s critical to “better coordinate between coalition forces, Iraqi security forces and concerned citizens,” as he calls the vigilante-style groups that have sprouted up across the country to fight extremists.

The comments reflect rising concerns about possible friendly fire killings that could threaten to undermine the U.S. strategy of seeking alliances with local Sunni and Shiite leaders to fill the vacuum left by a national police force that has been plagued by corruption allegations and infiltration by militants.

Incidents of shooting of civilians at checkpoints has drawn allegations by many, in Iraq and beyond, that U.S. troops and contractors are quick to fire and ask question later.

Such criticism was widespread after the March 2005 fatal shooting of an Italian intelligence officer at a checkpoint near Baghdad airport. The officer was traveling at night shortly after securing the release of a kidnapped Italian reporter, who was wounded along with an Italian driver when a U.S. soldier opened fire. The U.S. military has said the soldier acted appropriately in the incident.

After the Abu Lukah shooting, the so-called North of Hillah Awakening Council staged a three-day strike to register its anger over the loss of three of its members, but guards resumed their posts on Sunday.

“Such acts will create a gap between us and the Americans. We are trying to restore security in the area while the Americans are killing us,” Nabil Saleh, 37, said as he stood with his AK-47 slung over his shoulder at his post in Abu Lukah.

Jabar Hamid, a 33-year-old Shiite from the village, said the U.S. military had paid $2,500 to each family of the three men killed.

“It is a tragedy and regrettable thing,” he said.

In a bid to distinguish the recruits from potential militants, the groups have been given vests with reflective stripes, similar to those worn by traffic police in many countries. Others wear brown T-shirts with Iraqi hats similar to those worn by the national army.

Capt. John Newman, 31, of Columbus, Ga., said the soldiers believe they can discern volunteers from the insurgents.

“We’ve given them their road guard vests,” Newman said. “So, he’d better be wearing that vest if I see him carrying an AK-47.”

Lynch stressed the Americans are not arming the groups because the men already have weapons, primarily AK-47s that are legally permitted in Iraqi households.

“We are allowing the people of Iraq to secure their own areas and they are using their personal firearms to do that,” he said.

The southern belt of Baghdad is a mosaic of Sunni enclaves, such as Arab Jabour and Jisr Diyala, once al-Qaida havens, and all-Shiite strongholds, such as the town of Nahrawan. In that community, the country’s strongest Shiite militia, the Mahdi Army, has lately been overshadowed by rogue Shiite elements and “gangs,” as the U.S. military describes them.

Iraqi volunteers — both Sunnis and Shiites — mostly watch over their neighborhoods, guard mosques and man checkpoints. The theory is that, as natives to the area, they can better recognize foreign fighters and al-Qaida loyalists in their midst.

The tactic was first implemented in the Sunni western Anbar province, and later in Diyala, a province northeast of Baghdad.

Now it is being tested in Lynch’s territory, such as the wind-swept planes surrounding U.S. patrol base Hawks, 20 miles southeast of Baghdad — one of 36 small bases Lynch’s troops have built up as outposts in their region.

Prime Minister Nouri al-Maliki and other Shiite leaders have expressed concern over the American policy of sponsoring armed Sunnis, many of whom were likely former insurgents.

“Acceptance rules for these recruits should be within a legal framework so that we do not allow the emergence of new militias,” al-Maliki said Friday during a meeting with the new chairman of the Joint Chiefs of Staff Navy Adm. Mike Mullen.

The U.S. military says the ultimate goal is to bring the volunteers into the Iraqi security forces, which the Americans hope will be eventually able to take over the country’s security so they can go home.

Lynch insisted that every volunteer is nominated by tribal leaders and vetted by Americans with retina scans and fingerprinting. The serial numbers of their AK-47s also are logged.

“We know who they are, where they are,” Lynch said, adding that his region now has more 20,000 Sunnis and Shiites who have come forward to join the alliances.

He said the formation of such groups has been a major factor in the success achieved since his troops arrived in April as part of President Bush’s troop buildup. He cited a 64 percent reduction in attacks and a 60 percent drop in the number of local casualties, although he didn’t give specific figures.

Lynch also acknowledged the volunteer groups could become a problem later if they are not brought into the mainstream.

“They want recognition,” he said. “If they get a sense that they are not recognized or treated as legitimate, they could potentially go back to their rogue ways.”