People walk through Miami’s Overtown neighborhood on Wednesday. (Scott McIntyre/For The Washington Post)

When 10-year-old Alton Banks left the community swimming pool on the last day of his life, he walked past the elementary school where he had just finished fifth grade.

He passed a cheery banner that defined a beaten-down ­inner-city neighborhood trying to will itself into up-and-comingness: “Experience Overtown. Eat, Live, Work, Play.”

He walked past a fancy new apartment building under construction, then a long row of ragged homes and chickens clucking freely on sidewalks littered with crushed tall-boy beer cans in brown paper bags.

He arrived home on that hot afternoon, June 23, and climbed the concrete steps to his second-floor apartment across from the homeless people crowded under a highway overpass.

He started vomiting. His mother called an ambulance. And that evening he was dead, killed by a combination of heroin and fentanyl, a powerful synthetic opioid.

And no one yet has the slightest clue how the opioid crisis that has battered the nation with such ferocity ended up taking a happy, skinny little boy a month shy of his 11th birthday.

“We really don’t understand how this could possibly have ­happened,” said Patricia Ares-Romero, chief medical officer at Jackson Behavioral Health Hospital.

The pool at Gibson Park in Overtown. Alton Banks swam there the day he died of a fentanyl and heroin overdose. (Scott McIntyre/For The Washington Post)

Miami-Dade State Attorney Katherine Fernández Rundle made the preliminary toxicology results public this week as a plea for help. She raised the possibility that Alton came into contact with the drug at the pool or on the walk home in a neighborhood notorious for being Miami’s busiest illicit opioid marketplace. Authorities have no evidence to suggest that Alton came into contact with the drugs at home, she said.

Fernández noted that just a few tiny grains of fentanyl are enough to cause fatal overdoses, and it can be absorbed through the skin or breathed in. While Ares-Romero declined to comment on Alton’s situation, she said that it was conceivable that heroin could be absorbed into a person’s bloodstream if he or she came into contact with a mixture of heroin and fentanyl, an increasingly common mixture on Miami’s streets.

“It’s the nightmare that every parent fears,” Fernández said.

Signs on NW 13th Street in Overtown. (Scott McIntyre/For The Washington Post)

A grief counselor at Alton’s home Wednesday said his mother was not feeling up to speaking.

The opioid crisis — and the increasing prevalence of fentanyl, a painkiller 100 times more powerful than morphine — has ravaged the United States with a lethality not seen since the AIDS epidemic of a generation ago.

Opioids are best known as a scourge of white, working-class America from the Midwest to New England, but the nation’s big cities, too, have been increasingly brutalized. Miami-Dade County had about 100 opioid overdose deaths a year from 2005 to 2015, but that jumped to 229 last year, according to a recent report by a county task force.

Fentanyl — or one of its even more powerful “analogues,” or chemically similar variants — was detected in 376 overdoses in Miami-Dade County from 2014 to 2016, the task force report said.

Ares-Romero, who was a member of the task force, said overdose patients tell her that dealers are touting fentanyl’s lethality as a selling point.

“The dealers will tell people, ‘This stuff will kill you!’ And that’s attractive to them because they want to ‘chase that dragon’ and get that high,” she said.

The epicenter of Miami’s opioid and fentanyl ravages is Overtown, the storied African American neighborhood where Alton lived. Just north of the city’s bustling downtown, the neighborhood was disrupted decades ago by the construction of two intersecting, elevated superhighways that isolated the community and contributed to an economic decline.

A neighbor of Alton Banks’s family, Jessie Davis, outside of her home in the Overtown neighborhood. (Scott McIntyre/For The Washington Post)

Overtown also has been hard hit by crime and drugs. There are signs of rebirth and renovation amid the crumbling buildings, and walls are decorated with sumptuous murals. But the neighborhood still feels broken.

“If I had money, baby, I wouldn’t be here,” said Jessie Davis, 56, who lives in a four-unit apartment building next to Alton’s home. She has been there for 21 years, and she has nine children and seven grandchildren — some of whom spend their summer days swimming at the Gibson Park pool where Alton spent his last day.

Davis said drug dealers are common in Overtown, but she said the place still feels safe enough to let her children walk the streets during the daytime. “You really can’t shield them from everything,” she said.

Kenlisha Hubert, 18, concedes Overtown “is a tough neighborhood.” (Scott McIntyre/For The Washington Post)

Her youngest daughter, Kenlisha Hubert, 18, said that “it’s a tough neighborhood, but I’m used to it.” She said she once saw a woman overdose in her back yard, banging her head repeatedly against a piece of furniture until an ambulance arrived.

Jeffrey Mitchell, 57, who lives in the neighborhood and was riding his bike past Alton’s house Wednesday, said many of the Overtown drug dealers hire preteens to sell for them because youngsters can’t be tried as adults if caught.

“The dope boys got little kids selling drugs for them when they should be in school, getting an education,” he said. “Ten or 11 years old, selling drugs. That’s just as wrong as all outdoors.”

Ares-Romero, the doctor, said most of the overdose victims who arrive by ambulance from Overtown live somewhere else. She said people come from as far as West Palm Beach, 70 miles to the north, to find a quick fix on Overtown’s struggling street corners.

Asked how often his firefighter paramedics respond to overdoses, Capt. Douglas Reno of the Miami Fire Department Station 2 replied, “Every day — multiple times, sometimes seven or eight a day.”

The station, which dispatched the ambulance that took Alton to the hospital, sits in the heart of Overtown and handles calls that take the paramedics across the neighborhood’s socio-economic range, from underneath Interstate 395, where the homeless huddle on mattresses, to fancy new high-rise apartment buildings.

People walk through Overtown. (Scott McIntyre/For The Washington Post)

Just around the corner from the Gibson Park swimming pool, a large oak tree in a vacant lot is marked with a spray-painted white cross. It’s the spot where Kyle Dodds, 24, overdosed and died last September.

His mother, Cindy Dodds, 60, from Key Biscayne, said he collapsed after snorting a mixture of cocaine, heroin, fentanyl and carfentanil, a super-powerful opioid.

“There’s not a day down in that neighborhood where there’s not something bad going on,” Dodds said of Overtown, where she has become active in the community.

Dodds was a vocal advocate of a new Florida law, which takes effect in October, that allows prosecutors to bring murder charges against anyone who sells fentanyl that results in an overdose death.

She said her son’s descent into drug abuse started when he was prescribed oxycodone for a football injury when he was 15.

Ares-Romero noted that Florida long had a reputation for being a “pill mill” where people from out of state came to load up on prescription painkillers. When the state cracked down on that, many turned to heroin and other illegal street drugs.

Dodds has advocated for restoring a historic hotel close to where Kyle died, refashioning it into a welcome center and nonprofit hub. Most of all, she wants to create a memorial garden for Kyle.

She said she wants it to be “full of beautiful wildflowers, but not high enough that people can die in them.”

This cross is at the spot where Kyle Dodds, 24, died of an overdose of heroin and fentanyl in September 2016. (Scott McIntyre/For The Washington Post)

Joel Achenbach, Emma Ockerman and Wesley Lowery in Washington contributed to this report.