It took an alleged terror plot for federal authorities to pay attention to the Duka brothers, the three illegal immigrants who were accused this week of planning to attack soldiers at Fort Dix.

But the Yugoslavian-born siblings may have flown under the radar for a long time before that, going to school, working jobs, registering businesses - even repeatedly having run-ins with police - without their right to be in this country being questioned.

The fact that three of the six terrorism suspects rounded up Monday night were here illegally - and may have been for most of their lives - reignited criticism of an immigration system that has allowed an estimated 12 million foreigners to slip through America's cracks.

"We make it very easy for illegals to live here," said Mark Krikorian, executive director of the Center for Immigration Studies, a Washington think tank that favors tighter immigration restrictions. "Immigrants are people like everyone else. They respond to incentives. And the incentive here is, if you make it past the border, you're pretty much home free."

Congress is debating ways to address the situation with illegal immigrants, but has yet to settle a split between those who favor offering immigrants a path to citizenship and those who demand an enforcement crackdown and mass deportations.

The only thing that almost everyone can agree on is that the current system is not working.

Federal immigration enforcement officials cannot or will not say much about Eljvir, Shain and Dritan Duka's history in the United States, other than that they are in the United States illegally. They have not commented on how, when and why they came, and whether they ever sought or got papers to live here.

What is known is that the brothers, from the town of Debar in what is now Macedonia, arrived as children with their parents in 1984. A lawyer representing one of the brothers said the family entered across the Mexican border. They spent time in Texas and Brooklyn, N.Y., before moving to Cherry Hill in 1996.

It is unclear whether they sneaked into the country, applied for asylum, or entered on a visa.

But some immigration experts say it would be unusual for the family to have sneaked into the United States more than 20 years ago without being legal - or at least trying to legitimize their status - at some point.

"My guess is they have had some kind of complicated immigration history," said Linda Bosniak, a professor at the Rutgers University School of Law at Camden, who teaches immigration and citizenship law.

At least one family member appears to have been a legal resident for part of his stay. Dritan Duka, 28, held some sort of visa to be in this country, and it expired in June 2006, according to the New Jersey Motor Vehicle Commission, which suspended his driving privilege as a result.

The brothers' immigration status otherwise never seemed to affect their day-to-day lives.

Eljvir, 23, and Shain, 26, enrolled in Cherry Hill schools in 1996. Eljvir attended until the end of 10th grade, in 1999. Shain stayed until the end of 11th grade, in 1998.

School officials, who require a passport or birth certificate to register students, never would have known the boys' immigration status, said schools spokeswoman Susan Bastnagel. That's because schools are forbidden to inquire about immigration status in New Jersey, and a U.S. Supreme Court decision guarantees all children the right to an education regardless of whether they are here illegally.

The boys went on to get regular jobs. They worked at family-owned pizzeria and roofing companies, including several registered by their father, Ferik Duka. The father also was here illegally and was taken into custody by Immigration and Customs Enforcement agents the night of the raids that netted the three brothers.

(The Duka brothers' alleged coconspirators in the Fort Dix plot - Agron Abdullahu, Mohamad Shnewer and Serdar Tatar - also are foreign-born, but were legal U.S. residents, authorities said.)

Even the Dukas' dealings with New Jersey motor vehicle officials raised no red flags.

It's unclear what documentation Shain Duka had, but he managed to get a driver's license in 1999. Eljvir and Dritan Duka never had a license, but applied for - and got - driving permits on different occasions. Dritan Duka also got a nondriver identification card, perhaps with the help of an immigration visa that motor vehicle files say expired last year.

Michael Horan, a motor vehicle spokesman, pointed out that the brothers applied for their licenses and permits before the state introduced strict identification requirements and tough-to-counterfeit digital licenses in 2004.

"It's to prevent those not legally entitled to a driver license from getting one," Horan said. "Now we're recognized as one of the leaders in the country as far as having one of the most secure licenses."

Police, too, apparently didn't question the Dukas' immigration status in their numerous run-ins with the brothers.

According to New Jersey motor vehicle records, the three men were collectively stopped dozens of times by police, mostly in South Jersey, and racked up almost 50 citations for everything from speeding to careless driving and improper turning and passing.

All three brothers also were repeatedly cited for driving without a license or, in cases where they didn't have a license, driving while their driving privilege was suspended.

Police in the brothers' hometown of Cherry Hill said they were known for driving without a license and pulled over often as a result.

Police spokesman Lt. William Kushina said that if officers had trouble identifying someone and suspected he was here illegally, they could refer the case to federal authorities.

Officers also often run checks through a database that contains immigration information, Kushina said, but he could not say how often the Dukas were run through the database.

He also pointed out that unless the individual has an immigration record - for example, has been caught overstaying a visa, or has a warrant out for his arrest for skipping a deportation hearing - he may not be in the system.

"I could run them 100 times, but if there's no red flags in there, they could come back clear," Kushina said.

The fact that they didn't look or talk like Mexicans - who represent the largest segment of illegal immigrants - likely helped them fly under the radar, one immigration expert said.

The Dukas, in many ways, looked "American." They were light-skinned and spoke English with little or no accent. Eljvir called himself "Elvis" and Dritan went by "Tony."

"I think the race piece is important. As a practical matter, it's far more likely that someone pulled over that fits the profile of a Mexican landscape worker is likely to have an immigration inquiry put to him than someone who looks European," said Bosniak, the Rutgers professor.

In Cherry Hill, at least, the Dukas weren't considered foreigners at all - as Kushina pointed out, they "have lived here a long time."

"We knew who they were. They went to the schools," Kushina said. "They were locals."

Contact staff writer Jennifer Moroz at 609-989-8990 or jmoroz@phillynews.com.