From the outside, the houses in Braintree and Melrose may have looked like regular single-family suburban homes, but inside, prosecutors say, undocumented immigrants were toiling to pay off smuggling debts by growing marijuana as part of a multimillion-dollar illegal enterprise.

Seven Chinese nationals were charged on Tuesday in federal court in what prosecutors say was a scheme to use illegal immigrant labor to produce black market marijuana in non-descript homes in towns across Massachusetts and Maine. Six of the defendants have been arrested and one is still a fugitive, officials said.

“This case pulls back the curtain on a sprawling criminal enterprise that exploited our immigration system and our communities for personal gain,” said U.S. Attorney Leah B. Foley in a press release. “These defendants allegedly turned quiet homes across the Northeast into hubs for a criminal enterprise — building a multi-million-dollar black-market operation off the backs of an illegal workforce and using our neighborhoods as cover. That ends today.”

Prosecutors say the Chinese laborers were allegedly smuggled into the U.S. and confined to the homes. Their passports, they say, were confiscated and held by the operators of the criminal enterprise “until they repaid their smuggling debts.”

The defendants were charged with money laundering, alien smuggling and drug forfeiture among other things. Notably, they weren’t charged with labor trafficking, something that Amy Farrell, director of the School of Criminology and Criminal Justice at Northeastern University, said was likely also occurring.

Farrell noted that the lack of a trafficking charge will make it harder for the alleged victims to get help. A special visa is often provided to trafficking victims who are undocumented. T visas, as they are called, allow the victims to legally remain in the U.S. and assist in any prosecution.

She questioned why trafficking charges were not applied in this case.

“They were being exploited. It’s very clear that exploitation seems to rise to a level that certainly a labor trafficking case could have been made,'' Farrell told GBH News. ”I’m sure the prosecutors have information that we don’t have. But they probably won’t get any relief unless someone recognizes them as victims of forced illicit labor trafficking.“

Still, Farrell said it is not unusual that labor trafficking charges were not levied against the defendants in this case, an issue GBH News has reported in the past.

“That actually isn’t completely surprising. Like we see parallels within massage parlors in some ways that are probably quite similar where we see individuals that were probably knowingly smuggled into the United States, so they weren’t like kidnapped from anywhere else. That seems to be the case here. They owed a debt, and they worked in a massage parlor to pay off that debt. And when massage parlors get raided or they get shut down, those individuals go into the wind and they’re really hard to find. And that may be in the case here.”

This kind of enterprise, she said, has grown in recent years.

“Right now I’m in the middle of a big study looking at what we call forced criminality, which is a form of labor trafficking in which individuals are forced by a variety of different means or coerced into illicit economy work.”

Farrell said these kinds of cases are happening all around the country “and marijuana growing is certainly one of the areas where we’ve seen other prosecutions.”

She said workers are held in check by coercive means such as passport confiscation and threats of violence.

The defendants allegedly operated through a network of interconnected grow houses in Massachusetts and Maine and stayed in touch with each other through a list of marijuana cultivators and distributors with ties to the lead defendant, Jianxiong Chen, 39, of Braintree, with what they called an “East Coast Contact List,” according to the federal indictment.

Though the enterprise has been functioning since 2020, according to Foley, it was only recently uncovered by federal investigators.

A white suburban single-family home in Braintree was allegedly used by Chen as the base of operations. The house was searched in October of 2024 and $270,000 in cash was recovered, as well as a Porsche that was sitting in the driveway.

It is further alleged that co-conspirators concealed the marijuana and cash they were delivering to Chen inside the engine compartments of their vehicles. Federal prosecutors said the enterprise generated millions of dollars in profits.

Braintree Police Chief Timothy Cohoon told GBH News in an email that he knew nothing of the enterprise that was operating in his town and “had no involvement in this case (outside of courtesy notifications).”

In a safe located in one of the houses, investigators found passports belonging to the Chinese nationals that were coerced, according to the government, to cultivate the marijuana until they repaid the cost associated with smuggling them into the country.

Officials from the U.S. Attorney’s Office declined to say how many people were working in the homes or what happened to them.

The charge of conspiracy to manufacture, distribute and possess with intent to distribute marijuana provides for a sentence of up to five years in prison, at least two years of supervised release and a fine of up to $250,000. The charge of “bringing aliens into the United States” provides for a mandatory minimum sentence of three years and up to 10 years in prison, three years of supervised release and a fine of up to $250,000.

None of the defendants or their attorneys could be immediately reached for comment.