HUSAYBAH, Iraq — The Marines stationed at Camp Gannon, on the outskirts of this outlaw town where insurgents are thick on the ground, are used to being shot at. So when they recently heard AK-47 weapons fire and dozens of mortar blasts echoing throughout the town, they weren't surprised.

This time, however, they weren't the target.

"They were shooting at each other," said Capt. Frank Diorio, the camp's commanding officer.

From observation posts on the edges of the camp, Marines say they have watched insurgents lob dozens of mortar rockets at each other and engage in hours-long gunfights. And townspeople, troops here believe, have occasionally shot back.

Some Marines think the fighting is between groups aligned with foreign insurgent groups and those indigenous to the area.

Sitting at the intersection of the Syrian border and the Euphrates River, this ancient town of 30,000 people has for centuries been a crossroads for wanderers, traders and smugglers existing virtually independent of any national authority.

Even Saddam Hussein failed to tame much of the vast western Al Anbar province, with its oceans of sand and isolated Sunni Arab tribes.

Over the past two years, the U.S. military says, Husaybah has become a thoroughfare and training ground for insurgent fighters flowing in from Syria.

"The insurgents get training and finances here and then move east," Diorio said. "Now it seems a lot more are staying around. They want to make this area another Fallujah — but many of the locals don't want any part of it."

Lt. Ronnie Choe, the camp's intelligence officer, said many local people who initially fought alongside insurgents trickling across the border have become disillusioned with the insurgents' antagonistic behavior. Whether this is wishful thinking on the Marines' part or reality cannot be determined; it is too unsafe to venture into the town to interview locals.

"Tensions in Husaybah arose from foreign fighters coming here and staying here," Choe said. "Even the imams have been intimidated by the mujahedeen."

Last month, Choe said, insurgents kidnapped a cleric who had delivered a Friday sermon asking foreign fighters to stop attacking Americans from Husaybah because it put townspeople at risk when Marines returned fire. Choe also described how foreign fighters have hijacked Friday sermons.

"You'll hear one voice giving the sermon, and then someone else will get on," he said.

Choe noted that calls on the camp's tip line are increasing and said those contacts are his best source of intelligence.

Two weeks ago, troops raided a house in Husaybah and found a weapons cache. The family readily acknowledged that insurgents had hidden contraband in their home, Diorio said.

"They said, 'Yeah, they came and put weapons here,' " Diorio recalled. " 'And before they left they shot my son.' "

Husaybah's shift from being an insurgent thoroughfare to being a destination point for foreign fighters was evident during a large-scale attack on Camp Gannon last month.

In that attack, 30 to 40 insurgents fired Kalashnikov rifles and rocket-propelled grenades to divert attention from three vehicle bombs — a pickup truck, a large dump truck and a fire engine. The drivers of the two larger vehicles tried to breach the camp's inner security wall.

The attack failed, and only two Marines were slightly injured in the attack. But the high level of planning and coordination evinced foreign involvement and a large commitment of insurgents' time and resources, U.S. military officials said. Al-Qaida would later claim responsibility for the attack.

"That attack showed us that they have the capability to do a high-end attack," said Lt. Col. Timothy Mundy, the battalion commander for Camp Gannon and nearby Qaim. "It took a lot of time and effort to plan that attack, but it is not something that they can sustain."

Such attacks have resulted in few American casualties, a fact that has not been lost on insurgent detainees, said Capt. Tom Sibley, battalion intelligence officer.

"Many of these guys have been disillusioned," Sibley asserted. "Our detainees throughout this area tell us that." Sibley said that many detained Iraqi insurgents in Al Anbar province tell interrogators they are tired of resisting.

"A lot of them come from families where one or two of their brothers has been detained or killed," Sibley said. "Many of them used to have legitimate day jobs — they're trying to carry on with their regular lives and at the same time carry on the insurgency. They have wives ... children who miss them."

But on the other hand, hard-core insurgents are pressuring them to continue fighting, Sibley said. "The al-Qaida guys kill these people if they don't cooperate," he said.

The Camp Gannon Marines acknowledge that many residents of Husaybah are caught in the middle, watching both sides to see who has the most staying power. Diorio said the Marines would remain there until Iraqi security forces are established there later in the year.