The FBI has launched an investigation into how applicants with phony or questionable credentials were able to get Massachusetts massage therapist licenses, according to two people briefed on the probe, bringing upheaval to a board that is supposed to help prevent sex trafficking.

Investigators are looking into why employees of the state Board of Registration of Massage Therapy approved the applications of candidates who reported getting their training at Axiom Healthcare Academy in New Jersey, after the school closed in 2014, according to the two people. Axiom’s founder pleaded guilty in 2018 to charges he sold fake transcripts and phony graduation certificates to people working as prostitutes at massage parlors.

Since the investigation began, a massage board staff member has been suspended for licensing Axiom “graduates,’’ and the board’s executive director stepped down without explanation two weeks ago.

“The same staff has been there for years,’’ said a former massage board employee, expressing surprise at the failure to spot phony credentials. “I don’t know how they didn’t catch them.’’ The former employee requested anonymity out of fear of retribution from the owners of illicit massage spas.

Officials at the Division of Professional Licensure, which oversees the massage board and 37 others, acknowledged the existence of a federal investigation in January when they rejected a Globe public records request for documents related to the applications of hundreds of licensed massage therapists.

“Federal authorities have advised DPL that dissemination of the requested records at this time will jeopardize or prejudice investigative efforts because these documents are related and/or touch upon an ongoing investigation,’’ Kevin Scanlon, general counsel of the Office of Consumer Affairs and Business Regulation, wrote in a January e-mail.

A spokeswoman for the US attorney’s office also declined to comment. An FBI spokeswoman said she could neither confirm nor deny the existence of an investigation.

Last week, the Division of Professional Licensure belatedly released documents to the Globe in response to a request for records related to Axiom. The records show officials started suspending the licenses of several massage therapists with Axiom credentials in November — a month after the Globe requested the documents.

Carolyn Assa, spokeswoman for the state’s Office of Consumer Affairs and Business Regulation, said the Division of Professional Licensure is now conducting an internal review of all 9,000 massage therapy licensees.

“Public safety and license integrity are the Division of Professional Licensure’s top priority and the division has recently enhanced protocols to strengthen screenings for all applicants on a go forward basis, particularly for out-of-state schools,’’ Assa said.

“DPL staff is trained to look for signs of human trafficking in establishments and refers all suspected cases over to law enforcement for further investigation, as the division is not a law enforcement agency,’’ she said, adding that “the department investigates all complaints.’’

Since Jan. 1, Assa said, each out-of-state application has been reviewed twice, and applicants will not be approved until the board receives proof that the school is in good standing with its state licensing authority.

The massage board, created by the Legislature in 2006, was supposed to relieve cities and towns of the burden of regulating the industry. The board is authorized to scrutinize the backgrounds of therapists and spa owners alike before issuing licenses, which are required. The board can fine spas that don’t comply with the rules and can ultimately revoke their licenses.

But several police departments complain that the state actually undercuts local government efforts by licensing some people who are suspected of sex trafficking. In addition, they say, the state board’s rules supercede local regulations, making it hard for cities and towns to set their own standards for massage parlors. In fact, a 2008 letter from the Division of Professional Licensure told cities and towns they could no longer regulate or license massage therapists.

The town of Oxford, for example, tried to crack down on unlicensed massage parlors that officials believed were offering sex to customers, based on complaints from neighbors and the town’s own investigations.

Even though state licenses are required, violators typically face only fines. So, Oxford police and the board of health worked together to create their own rules, requiring the town’s three unlicensed massage parlors to undergo frequent inspections, identify customers and employees, and submit to criminal background checks.

But the owners found an easy way to bypass the local bylaw: They got licensed by the state.

“We went to do the inspections and we were trumped by the state licenses,’’ said Oxford police Chief Anthony Saad. “We couldn’t do anything.’’

The spa operators have denied any wrongdoing.

A Globe review of the state’s roster of massage therapists reveals hundreds whose credentials, addresses, or backgrounds could have raised questions for regulators.

For instance, more than 300 current Massachusetts massage license holders list Flushing, N.Y., as their home address. Law enforcement officials and experts have described Flushing as the hub of sex trafficking in the United States — where young women enter the country and are then dispatched to other cities. The former massage board employee said the board did not single out these applicants for increased scrutiny because they felt that would be discriminatory.

A major sex trafficker convicted in Florida last year of operating illegal Asian massage parlors across the country has at least one licensed massage establishment in Massachusetts, according to federal court records. David C. Williams, who pleaded guilty in November to human trafficking, was cited in court records as associated with Braintree Massage, which operates on Commercial Street and is licensed by the state. Williams, prosecutors said, used other people’s names on official records to conceal his ownership.

A woman there answered the phone “Hello?’’ but didn’t speak English and wouldn’t take a message.

Even some women previously convicted of prostitution have valid Massachusetts massage licenses. Yu Yun Chen of Quincy, for example, was charged in 2013 with engaging in prostitution and operating an unlicensed massage business in Andover. Police said she ran a similar operation in Danvers. Chen admitted sufficient facts, akin to a guilty plea, in 2014 in Lawrence District Court and was placed on probation for six months. In 2017, she received a license from the state massage board, records show. Old misdemeanor convictions do not disqualify applicants for state licenses.

Earlier this month, the Division of Professional Licensure disciplined one employee. FeiYan Chen was suspended for three days for approving the licenses of applicants who claimed to have graduated from Axiom, according to records and a former DPL official.

In an e-mailed message, Chen said she would speak to her supervisor and the lawyer for the board before deciding whether to comment.

Records show Chen approved the applications of at least eight people who claimed to have graduated from Axiom, the now-closed school in Bergen, N.J. Its founder, Naresh Rane, pleaded guilty in 2018 in federal court in Newark to using interstate facilities to promote prostitution. He is currently awaiting sentencing. His lawyer, David A. Schwartz, declined to comment.

Rane sold fake school transcripts and graduation certificates for between $1,000 and $2,600 each, “knowing that these documents would be used to facilitate the prostitution business,’’ court records show. These documents enabled workers to obtain state massage licenses, “which could be displayed at massage parlors offering prostitution services’’ to create the appearance of legitimacy, prosecutors said.

In Massachusetts, most of the applicants claimed to have attended Axiom in 2015 or later — although the school closed in 2014. Rane wrote a reference for one of the applicants.

The former massage board employee said the massage board also licensed therapists who graduated from other schools that had lost their accreditation, or were closed or unapproved. Sometimes required references were clearly fabricated, as well, claiming, “I went to church with her in New York’’ at a time when the applicant reported living in California, the employee said.

The federal probe comes as the Division of Professional Licensure is already facing heavy criticism for allowing applicants with long criminal records to obtain other types of state licenses. In December, the Globe reported that some dangerous sex offenders, including an electrician who served time for assaulting a boy he coached, and another electrician who served time for murder, were able to get electrician licenses.

“There has to be some sort of vetting process that happens so people feel safe. Any breakdown in that chain causes a problem. That’s what we’re seeing now,’’ said Senator Paul Feeney, chairman of the Joint Committee on Consumer Protection and Professional Licensure. The panel held a sometimes contentious hearing with DPL leaders in January after the Globe story.

Leadership of the division has changed substantially in recent weeks.

In Oxford, officials have tried to for years to regulate or close three local massage parlors, including one where they arrested a massage therapist for soliciting an undercover officer in 2010.

The woman, Yim Wai Kuen, was charged with indecent assault and battery on a person over 14, a felony, and sexual conduct for a fee, a misdemeanor. She defaulted and a warrant was issued in 2011.

Oxford police say they would welcome a more vigilant massage board, or a new system of oversight, where cities and towns could also license or regulate massage businesses. The local bylaw, they said, should trump a state license.

“It’s a public safety nightmare,’’ said Oxford police Lieutenant William Marcelonis. “We don’t even know their real names. They’re being shipped in and shipped out.’’

“We used to get on the girls for not having licenses, but then the state in their infinite wisdom decided to pass legislation that says they’re in charge of the licensing and there’s nothing we can do to enforce it. So how effective do you think that might be?’’ he said.

Andrea Estes can be reached at andrea.estes@globe.com.