Mexican drug cartels whose operatives once rarely ventured beyond the U.S. border are dispatching some of their most trusted agents to live and work deep inside the United States _ an emboldened presence that experts believe is meant to tighten their grip on the world's most lucrative narcotics market and maximize profits.

If left unchecked, authorities say, the cartels' move into the American interior could render the syndicates harder than ever to dislodge and pave the way for them to expand into other criminal enterprises such as prostitution, kidnapping-and-extortion rackets and money laundering.

Cartel activity in the U.S. is certainly not new. Starting in the 1990s, the ruthless syndicates became the nation's No. 1 supplier of illegal drugs, using unaffiliated middlemen to smuggle cocaine, marijuana and heroin beyond the border or even to grow pot here.

But a wide-ranging Associated Press review of federal court cases and government drug-enforcement data, plus interviews with many top law enforcement officials, indicate the groups have begun deploying agents from their inner circles to the U.S. Cartel operatives are suspected of running drug-distribution networks in at least nine non-border states, often in middle-class suburbs in the Midwest, South and Northeast.

"It's probably the most serious threat the United States has faced from organized crime," said Jack Riley, head of the Drug Enforcement Administration's Chicago office.

The cartel threat looms so large that one of Mexico's most notorious drug kingpins _ a man who has never set foot in Chicago _ was recently named the city's Public Enemy No. 1, the same notorious label once assigned to Al Capone.

The Chicago Crime Commission, a non-government agency that tracks crime trends in the region, said it considers Joaquin "El Chapo" Guzman even more menacing than Capone because Guzman leads the deadly Sinaloa cartel, which supplies most of the narcotics sold in Chicago and in many cities across the U.S.

Years ago, Mexico faced the same problem _ of then-nascent cartels expanding their power _ "and didn't nip the problem in the bud," said Jack Killorin, head of an anti-trafficking program in Atlanta for the Office of National Drug Control Policy. "And see where they are now."

Riley sounds a similar alarm: "People think, `The border's 1,700 miles away. This isn't our problem.' Well, it is. These days, we operate as if Chicago is on the border."

Border states from Texas to California have long grappled with a cartel presence. But cases involving cartel members have now emerged in the suburbs of Chicago and Atlanta, as well as Columbus, Ohio; Louisville, Kentucky; and rural North Carolina. Suspects have also surfaced in Indiana, Michigan, Minnesota and Pennsylvania.

Mexican drug cartels "are taking over our neighborhoods," Pennsylvania Attorney General Kathleen Kane warned a legislative committee in February. State Police Commissioner Frank Noonan disputed her claim, saying cartels are primarily drug suppliers, not the ones trafficking drugs on the ground.

For years, cartels were more inclined to make deals in Mexico with American traffickers, who would then handle transportation to and distribution within major cities, said Art Bilek, a former organized crime investigator who is now executive vice president of the crime commission.

As their organizations grew more sophisticated, the cartels began scheming to keep more profits for themselves. So leaders sought to cut out middlemen and assume more direct control, pushing aside American traffickers, he said.

Beginning two or three years ago, authorities noticed that cartels were putting "deputies on the ground here," Bilek said. "Chicago became such a massive market ... it was critical that they had firm control."

To help fight the syndicates, Chicago recently opened a first-of-its-kind facility at a secret location where 70 federal agents work side-by-side with police and prosecutors. Their primary focus is the point of contact between suburban-based cartel operatives and city street gangs who act as retail salesmen. That is when both sides are most vulnerable to detection, when they are most likely to meet in the open or use cellphones that can be wiretapped.

Others are skeptical about claims cartels are expanding their presence, saying law-enforcement agencies are prone to exaggerating threats to justify bigger budgets.

David Shirk, of the University of San Diego's Trans-Border Institute, said there is a dearth of reliable intelligence that cartels are dispatching operatives from Mexico on a large scale.

"We know astonishingly little about the structure and dynamics of cartels north of the border," Shirk said. "We need to be very cautious about the assumptions we make."

In Mexico, the cartels are known for a staggering number of killings _ more than 50,000, according to one tally. Beheadings are sometimes a signature.

So far, cartels don't appear to be directly responsible for large numbers of slayings in the United States, though the Texas Department of Public Safety reported 22 killings and five kidnappings in Texas at the hands of Mexican cartels from 2010 through mid- 2011.

Still, police worry that increased cartel activity could fuel heightened violence.

In Chicago, the police commander who oversees narcotics investigations, James O'Grady, said street-gang disputes over turf account for most of the city's uptick in murders last year, when slayings topped 500 for the first time since 2008. Although the cartels aren't dictating the territorial wars, they are the source of drugs.

Riley's assessment is stark: He argues that the cartels should be seen as an underlying cause of Chicago's disturbingly high murder rate.

"They are the puppeteers," he said. "Maybe the shooter didn't know and maybe the victim didn't know that. But if you follow it down the line, the cartels are ultimately responsible."

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