ST. LOUIS >> Clara Walker, a mother of nine and grandmother of eight, was peering out the window of her home three years ago after hearing what she initially thought were gunshots from a television crime show.

But at that moment, Anthony Jordan, who authorities say was a gang enforcer known as “Godfather,” was spraying gunfire on the street outside, and two bullets struck Walker, killing her.

“St. Louis is a dangerous place right now,” Johnny Barnes, Walker’s longtime boyfriend, said during a recent interview. “It’s all around us.”

The death of Walker was linked by authorities to a violent St. Louis street gang with ties to a Mexican drug cartel that in the past has supplied marijuana and cocaine throughout the Midwest.

In recent years, however, Mexican traffickers have inundated the St. Louis area with a new, potent form of heroin, drastically reducing prices for the drug and increasing its strength to attract suburban users.

The dispersal of the cheap heroin has led to a surge in overdoses, addiction and violence in cities across the country.

Besides St. Louis — where the problem is particularly acute — Chicago, Baltimore, Milwaukee and Philadelphia have attributed recent spikes in homicides in part to an increase in the trafficking of low-cost heroin by Mexican cartels working with local gangs.

“The gangs have to have a lot of customers because the heroin is so cheap,” said Gary Tuggle, the Drug Enforcement Administration’s chief in Philadelphia, who observed the same phenomenon while overseeing the agency’s Baltimore office. “What we are seeing is these crews becoming more violent as they look to expand their turf.”

To attract customers, the cartels — usually through a local surrogate — instruct gangs to sell the drug at prices as low as $5 for each button (about one-tenth of a gram of powdered heroin, which could last a novice user an entire day). At times, the gangs distribute free samples, according to agents with the Drug Enforcement Administration.

The purity level of heroin seized by drug agents on the streets of U.S. cities has grown significantly in recent years, federal officials say, rising to 50 percent from 5 percent in St. Louis in the past several years, and as high as 90 percent in Philadelphia.

In a trend mimicked in large cities nationally, many of the heroin consumers in St. Louis are young whites in their 20s, who drive into the city from suburbs and distant rural areas, police say. And while most heroin overdose victims here are white, nearly all of the shooting victims and suspects in St. Louis this year have been black men and boys, police data shows.

“What I’m seeing at street level are violent disputes about money owed around heroin debts, with sometimes the dispute being about money, and sometimes about drugs,” said D. Samuel Dotson III, police chief of St. Louis.

In 2014, St. Louis had the highest homicide rate of any city with more than 100,000 people. Its 157 homicides that year increased 18 percent in 2015 to 188, and while the rate has slowed in the initial months of this year, St. Louis is again on pace to be among the nation’s most dangerous big cities.

The heroin problem has been difficult for the city’s leaders to ignore. Those who have succumbed to the drug include a nephew of Steve Stenger, the St. Louis County executive, who died from an overdose in 2014. A brother of Mayor Francis Slay was arrested on a charge of heroin possession in 2012, and the stepson of Jennifer Joyce, the city’s top prosecutor, was arrested on the same charge last month.

“These heroin addicts are daughters, sons, husbands, wives or, in my case, a brother,” Slay told reporters last month.

Mexican cartels, including the Sinaloa cartel, which dominates the supply of illegal drugs throughout the Midwest, have generally not engaged directly in violence in St. Louis, law enforcement officials here say. But cartel lieutenants have sought to incite rivalries among street crews, authorities say.

The drug syndicate that federal authorities say mistakenly killed Walker four days after Christmas in 2013 was led by José Alfredo Velazquez, a Mexican-born businessman who speaks little English, and Adrian Lemons, who has a drug arrest record in St. Louis dating to the 1990s, according to a federal indictment and other court records. Officials say Velazquez has been linked to the Sinaloa cartel, long headed by Joaquín Guzmán Loera, the drug kingpin known as El Chapo, who has repeatedly escaped from Mexican prisons only to be caught again.

The partnership between Velazquez, 55, and Lemons, 38, began around 2012, primarily as a cocaine-dealing operation, officials said.

But as tastes in the area changed, the operation began to sell more heroin.

For four years, the enterprise proved nearly unstoppable, law enforcement officials contend. While Velazquez and Lemons focused on the logistics — racking up millions of dollars in heroin and cocaine sales — their street crew, including Jordan, 30, protected their turf by killing at least 17 people, including Walker, federal prosecutors say.

After a two-year investigation, the enterprise was finally dismantled in January, when 18 members of the gang — including Velazquez, Lemons and Jordan — were indicted on a variety of charges, including murder and drug trafficking. Trial dates have not been set.

Dotson said the local gang had been responsible for so much violence in the city that the arrests might lead to a significant reduction in violent crime, which rose 7.8 percent in 2015, including an 18.2 percent increase in homicides.

Lawyers for Velazquez and Lemons declined to comment.

Despite the indictments, heroin continues to be sold openly in the mainly black neighborhoods of north St. Louis once dominated by the group.

“They call and we tell them what time and this spot,” said a lanky 17-year-old who was selling heroin recently, and who gave his nickname as “B.” “If they have the right money, it’s right.”

© 2016 The New York Times Company