He knocked on the door of the squalid basement apartment, looking for a young couple. Their baby girl had been stopped at an airport thousands of miles away, and it wasn't her first suspicious trip.

The 8-month-old had already traveled to Panama and London five times. The latest trip had ended abruptly with an arrest at Heathrow Airport.

"Your baby was with a woman who was caught with drugs," U.S. Customs agent Pete Darling told the parents coolly. "Can you folks tell me what's going on?"

Calmly -- too calmly, he thought -- the couple claimed their child had been taken from a babysitter's house and they had filed a kidnapping report.

Darling noticed the parents appeared sickly and their apartment was a mess: dirty dishes in the sink, cardboard boxes on the floor, the smell of marijuana in the air.

Darling had begun to unravel an international drug smuggling ring -- a multimillion-dollar business that stretched from fleabag hotels in Panama to the Bronx to the industrial heart of England.

And it ran through one of the poorest pockets of Chicago, the drug-ridden, decaying neighborhood where Darling now stood -- the place the smugglers turned to for a precious commodity:

Babies.

As he prowled the terminal at Atlanta's Hartsfield International Airport, Mike McDaniel was already suspicious.

It was a few days after Darling's interview and McDaniel, a roving Customs inspector nearly 600 miles away, was looking for Chicago-tagged luggage on the carousels.

For weeks, he had encountered women, sometimes with babies, passing through en route to Chicago, who claimed they had visited husbands or boyfriends in the military in Panama.

Their stories didn't wash. The hotels they named were nowhere near military bases -- and in rough neighborhoods that didn't cater to Americans.

McDaniel happened to know that because he had served five years in Panama with the U.S. Army. And he had grown up near Chicago, so he realized the women lived in the same neighborhood.

McDaniel had done some luggage searches, but nothing turned up. Still, his suspicions remained.

Then he stopped Donna Washington, who said she had taken her grandson to see his father, stationed in Panama for the Army. But she wasn't able to tell him her son's address or rank.

When McDaniel asked to look inside her luggage, she blurted out that her bag was heavy. For him, that was a "tell," an inadvertent signal from a smuggler indicating where contraband is located.

Inside were six large cans of liquid baby formula and a seventh, smaller one.

McDaniel shook them and one rattled. Something solid was inside.

He suspected drugs. When he put the cans on a scale, the weights didn't match their labels. An X-ray showed the can with solid pieces contained 11/2-inch pellets.

McDaniel opened a different can and tested the liquid in a tube, and the contents turned blue. That meant cocaine.

He opened the can with solid pieces, broke off a piece of pellet and repeated the test. The contents turned green. That meant heroin.

Washington feigned surprise.

But her attitude turned indignant as McDaniel picked up the baby's bottle, twisted off the cap and sniffed it.

"What kind of person do you think I am?" she asked.

Customs agents now had two arrests, in London and Atlanta, with the same pattern.

Both women carried other people's babies and lived in Englewood, the depressed neighborhood on Chicago's South Side. And both stood accused of smuggling drugs in the same way: in infant formula cans, using infants as decoys.

Darling, a newcomer to Chicago and the Customs Service, started piecing together the puzzle. It was 1999, and this was his first big case. "I was looking to make my mark," he says.

Accompanied by a Chicago police officer, Darling returned to the couple whose baby had been stopped in London.

"You guys have got to tell me the truth," he told them. "This is real serious. It is not going to go away."

The parents, drug-addicted and HIV-positive, confessed, telling Darling a story he would come to hear many times: A neighborhood woman, Selina Johnson, had asked to be their baby's godmother, promising free milk and clothes for the child.

Johnson was more than six feet tall, charismatic and formidable: As the so-called first lady of the Sisters of the Struggle -- a female auxiliary of the Gangster Disciples street gang -- she could deal drugs in her neighborhood with impunity.

The couple told Darling that when their baby was 3 weeks old, they allowed Johnson to take her for a few days -- not even asking where they were going.

They eventually admitted they had "rented" their baby to be used as a decoy for international drug smugglers. The going rate: about $200-$400 or a small amount of marijuana per trip.

Other women, too, had taken their baby, they said.

"What do you mean?" the surprised agent asked.

They rattled off names: Kim, Janice, Nae-Nae. Sometimes they didn't even know precisely where the women lived.

Darling scribbled away on his notepad, his mind racing with a new reality: This drug ring was much bigger than he thought.

A paper trail would provide many clues.

Darling and federal prosecutor Scott Levine spent months poring over stacks of customs records and airline tickets, tracking the couriers' travels.

The smugglers flew from Panama City, and Montego Bay and Kingston, Jamaica, to Chicago, New York, London and Birmingham, England, bringing in more than 100 kilos of cocaine and six kilos of heroin. The couriers were paid up to $4,000 a trip; some also received drugs.

Most of the drugs they carried were concealed in formula cans that the smugglers figured would escape detection by drug-sniffing dogs. Cocaine was liquefied in Panama and injected into the can, which was then resealed and the label reattached.

Small cans could bring big cash.

A kilo of cocaine (about three cans' worth) that cost $5,000 in Panama could reap $20,000 or more in the United States and double that in England. Once it was cooked into crack and sold as dime bags, the profit multiplied by several times.

Jamaicans, Colombians, Panamanians and Americans all participated in the conspiracy. Fake passports and drivers licenses were obtained, and the couriers, many of them addicts themselves, took their own children or carried "rented" babies on dozens of trips -- a scam, says Levine, that posed extraordinary dangers.

"Can you imagine," the prosecutor fumes, "a drug addict from Chicago traveling in a foreign country where she does not even speak the language, taking care of a baby she has never seen, attempting to score some heroin . . . while she waits for cocaine-filled baby formula cans to arrive?"

That happened to the child identified in court records as "Baby 8" -- the little girl behind Darling's first trip to Englewood.

On one trip, she was handed over to a stranger's family by an addict who was robbed while looking for heroin; on another, she was left alone in a slum hotel by a courier who went on a beer run.

On her first trip, the baby -- then just 3 weeks old -- was deposited in an empty bathtub behind closed doors because she wouldn't stop crying.

One smuggler declared her too small and sickly to be a decoy. But he wasn't worried about her health.

He feared she'd catch the eye of Customs officials.

Pete Darling would travel to London, Atlanta and New York, but many answers he was seeking came from the streets of Englewood.

Climbing up crumbling steps and entering roach-infested apartments, he could not ignore the poverty that shaped the lives of the women and babies.

"This wasn't just dealing with bad guys," he says. "This was dealing with human beings struggling every day. . . . My heart just kind of broke when I saw some of the conditions they were living in."

Darling and his frequent partner, Billy Warren, an agent for the Drug Enforcement Administration, were outsiders. Warren was a former Kansas City cop; Darling had worked 121/2 years as a government investigator in Massachusetts, and his pronounced Boston accent frequently prompted the same question: "You're not from around here, are you?"

The agents discovered the truth often trickled out like water from a leaky faucet.

"Look," Darling would say on the third or fourth interview, "if you left some things out and want to bring them up now . . . I won't be offended if your story changes."

Many couriers were remorseful; one sobbed and repeatedly asked for God's forgiveness. Others were hostile: One woman kicked Darling out of her home.

Undeterred, the investigators kept digging, once tracking a woman based solely on her nickname -- "Punkin." She quickly confessed.

"Everyone has a button to push and you have to figure out what it is," Warren says.

But both agents also knew how important it was to win the confidence of the women. And that took time.

"I could spend an hour holding someone's hand," says Darling, who had learned the value of endurance as a marathon runner. "I would go through a very long empathetic speech . . . saying, 'We're not after the little people. We're going after the big people.' "

And they did.

Taking down a drug ring is like dismantling a pyramid, stone by stone, from the bottom up.

That's how Levine and fellow prosecutor David Hoffman, both veterans of the drug wars, cracked this far-flung operation. First, they flipped the baby-carrying couriers, then worked their way up.

It wasn't long before several couriers had confessed and two leaders -- Troy Henry and Orville Wilson, both Jamaicans -- were cooperating. Wilson, in turn, told prosecutors the formula cans were the brainchild of Clacy Watson Herrera, a Colombian charged with supplying most of the drugs.

For Levine and Hoffman, there were scores of interviews to conduct, and the occasional humorous detour.

One woman insisted she had traveled to Panama only to see the Panama Canal. Stepping out of his office, Levine asked her to compare the 50-mile-long canal with his 60-foot-long hallway. Without hesitation, she declared them the same size.

Mostly, though, the case was deadly serious. It was sheer luck, Levine couldn't help thinking, that none of the 22 babies was injured or mistakenly given the cans filled with drugs.

It would take 21/2 years to make all the arrests.

The last was Selina Johnson, the recruiter, who was apprehended at her grandmother's home as about a dozen agents, including Darling and Warren, stormed upstairs.

They found Johnson -- who had swallowed 20 to 30 dime bags of crack to hide evidence -- sitting on a bed, resigned, while several children huddled nearby crying.

Darling put his gun away and gently rubbed the back of his hand on one little girl's face. "It's going to be okay," he said.

Over the next two years, 48 defendants pleaded guilty, including Johnson, who received a 10-year sentence.

The couriers were sentenced to five to 10 years in prison; the parents who rented their babies, between 10 months and eight years. The only person who stood trial received a life sentence.

One last defendant, a leader who obtained some of the drugs and organized several Jamaican trips, awaits sentencing, scheduled for Wednesday.

Three men remain fugitives, and Herrera is serving a 72-month sentence in Panama for drug trafficking on an unrelated case. Prosecutors are trying to extradite him to Chicago.

Pete Darling was there at the end of the case, just as he was in the beginning.

He appeared at the sentencing of three couriers -- to testify on their behalf.

"I wanted to help those girls who tried to help themselves . . . to send a message that I'm not here all the time to hurt people," he says.

One woman Darling helped was Kim Washington, who had thrown him out of her apartment when he came to ask about her mother, Donna, who had been arrested in Atlanta.

In the four years since, Kim Washington had kicked drugs and found her first steady job.

"I was killing myself," she told the judge. ". . . I thank Pete Darling for arresting me. . . . He saved my life because I couldn't do it myself."

She was sentenced to 44 months in prison. Afterward, Darling embraced her family members.

Darling recently transferred to his native Boston, where he remains a Customs and Border Protection agent -- a job he sees as more than enforcing laws.

"I did some good in this case," he says. "I saved a couple of people from going down the road of destruction and possibly death. And there's the babies -- that's what bothered me so much. You have to have compassion in this job or you really shouldn't be doing this."

U.S. Customs agent Peter Darling stands behind some of the infant formula cans that contained drugs. The smugglers carried babies, often "rented" from addicted parents, to give a cover for the formula.Customs inspector Michael McDaniel's suspicions bore out when he discovered drugs in the baby formula cans.