CHICAGO — 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.

“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.

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.