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, Ky., 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."

Statistics from the DEA suggest a heightened cartel presence in more U.S. cities. In 2008, around 230 American communities reported some level of cartel presence. That number climbed to more than 1,200 in 2011, the most recent year for which information is available, though the increase is partly due to better reporting.

Dozens of federal agents and local police interviewed by the AP said they have identified cartel members or operatives using wiretapped conversations, informants or confessions. Hundreds of court documents reviewed by the AP appear to support those statements.

"This is the first time we've been seeing it -- cartels who have their operatives actually sent here," said Richard Pearson, a lieutenant with the Louisville Metropolitan Police Department, which arrested four alleged operatives of the Zetas cartel in November in the suburb of Okolona.

People who live on the tree-lined street where authorities seized more than 2,400 pounds of marijuana and more than $1 million in cash were shocked to learn their low-key neighbors were accused of working for one of Mexico's most violent drug syndicates, Pearson said.

One of the best documented cases is Jose Gonzalez-Zavala, who was dispatched to the U.S. by the La Familia cartel, according to court filings.

In 2008, the former taxi driver and father of five moved into a spacious home at 1416 Brookfield Drive in a middle-class neighborhood of Joliet, southwest of Chicago. From there, court papers indicate, he oversaw wholesale shipments of cocaine in Illinois, Wisconsin and Indiana.

Wiretap transcripts reveal he called an unidentified cartel boss in Mexico almost every day, displaying the deference any midlevel executive might show to someone higher up the corporate ladder. Once he stammered as he explained that one customer would not pay a debt until after a trip.

"No," snaps the boss. "What we need is for him to pay."

The same cartel assigned Jorge Guadalupe Ayala-German to guard a Chicago-area stash house for $300 a week, plus a promised $35,000 lump-sum payment once he returned to Mexico after a year or two, according to court documents.

Ayala-German brought his wife and child to help give the house the appearance of an ordinary family residence. But he was arrested before he could return home and pleaded guilty to multiple trafficking charges. He will be sentenced later this year.

Socorro Hernandez-Rodriguez was convicted in 2011 of heading a massive drug operation in suburban Atlanta's Gwinnett County. The chief prosecutor said he and his associates were high-ranking figures in the La Familia cartel -- an allegation defense lawyers denied.

And at the end of February outside Columbus, Ohio, authorities arrested 34-year-old Isaac Eli Perez Neri, who allegedly told investigators he was a debt collector for the Sinaloa cartel.

An Atlanta attorney who has represented reputed cartel members says authorities sometimes overstate the threat such men pose.

"Often, you have a kid whose first time leaving Mexico is sleeping on a mattress at a stash house playing Game Boy, eating Burger King, just checking drugs or money in and out," said Bruce Harvey. "Then he's arrested and gets a gargantuan sentence. It's sad."

Because cartels accumulate houses full of cash, they run the constant risk associates will skim off the top. That points to the main reason cartels prefer their own people: Trust is hard to come by in their cutthroat world. There's also a fear factor. Cartels can exert more control on their operatives than on middlemen, often by threatening to torture or kill loved ones back home.

Danny Porter, chief prosecutor in Gwinnett County, Ga., said he has tried to entice dozens of suspected cartel members to cooperate with American authorities. Nearly all declined. Some laughed in his face.

"They say, `We are more scared of them (the cartels) than we are of you. We talk and they'll boil our family in acid,"' Porter said. "Their families are essentially hostages."

Citing the safety of his own family, Gonzalez-Zavala declined to cooperate with authorities in exchange for years being shaved off his 40-year sentence.

In other cases, cartel brass send their own family members to the U.S.

"They're sometimes married or related to people in the cartels," Porter said. "They don't hire casual labor." So meticulous have cartels become that some even have operatives fill out job applications before being dispatched to the U.S., Riley added.

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."