Jessica Pastore, an MGB spokesperson, described the letter as without merit, saying, “We are confident that Mass General Brigham’s actions are fully consistent with our obligations as a nonprofit charitable organization.”

Among the trustees whom the union focused on are John Fish, chief executive of Suffolk Construction, Jonathan Kraft, president of the Kraft Group, and Phillip Ragon, founder and chief executive of the technology company InterSystems. Those companies received a combined $59.4 million in construction, leasing, and technology contracts, respectively, from MGB in fiscal 2024, the union said, citing IRS filings.

The union also asked Campbell to review the decision by MGB’s board to pay the health system’s chief executive, Dr. Anne Klibanski, nearly $9.2 million in 2024, a roughly 9 percent increase from the year prior. She is the top-paid health system executive in Massachusetts.

“MNA requests an assessment of whether the identified conduct is consistent with MGB’s obligations as a public charity and whether corrective action is necessary,” said the letter from Peter J. Duffy, a Boston lawyer hired by the union. Depending on the attorney general’s findings, Duffy wrote, the appropriate action might include “recovery of improperly diverted funds.”

Pastore said MGB has a rigorous conflict-of-interest policy that requires board members to disclose potential conflicts and recuse themselves from any matter in which they have a personal or business interest. Projects such as the construction that Suffolk has done for the health system are put out to competitive bidding, she added.

“Overall, Suffolk Construction has been awarded 5 percent of capital projects at Mass General Brigham’s hospitals since 2018,” she said.

As for Klibanski’s salary, Pastore said, the chief executive’s total compensation of $8.4 million in 2023 ranked well below the chief executives of rival health systems in the country, including Charlotte, N.C.-based Advocate Health ($30.9 million), New York Presbyterian ($26.3 million), and NYU Langone ($20.7 million). She added that when compared with peer institutions, Klibanski’s compensation consistently falls in the bottom third of the market.

Spokespersons for Fish, Kraft, and Ragon did not respond to requests for comment. Campbell’s office also did not provide a comment beyond confirming it received the letter.

Through the years, Suffolk Construction has had numerous multimillion-dollar contracts with MGB, including for serving as general contractor for the health care system’s 900,000-square-foot Assembly Row administrative campus in Somerville, says the union. In 2024, Suffolk received nearly $49 million from MGB for construction services, according to the IRS filing cited by the union.

Kraft has had contracts, including for leasing property at Patriot Place in Foxborough to MGB, the union says, although Kraft joined the board in 2010, after the deal was signed. In 2024, Kraft companies received $9.7 million for a lease and for marketing, IRS documents indicate.

Ragon’s company, InterSystems, has provided technology products and services to MGB. The company received $709,000 in 2024 from MGB, according to IRS documents.

Matthew McCoy, an associate professor of medical ethics and health policy at the University of Pennsylvania, reviewed the letter the nurses union sent to the attorney general. He said the union “makes a pretty compelling case” that the three trustees have conflicts of interest as executives and trustees.

That doesn’t preclude them from serving on MGB’s board, McCoy emphasized, but the health care organization must take precautions to avoid self-dealing. That includes: prohibiting board members from voting on contracts that could benefit their companies; soliciting multiple bids for projects that might involve trustees’ businesses; and providing transparency about any contracts MGB signs.

The lawyer for the nurses union wrote to Campbell’s office as a major strike by the nurses looms at Brigham. After a bitter months-long stalemate with hospital management, the union said Friday that Brigham’s nurses will walk out starting next Wednesday.

In contrast to the one-day strike by the nurses union at Brigham on July 8, this walkout is slated to be indefinite. That would make it far more disruptive to patients, the hospital, and nurses, who expect to forgo paychecks and lose Brigham’s contribution to their health insurance premiums.

The dispute is largely over wages, though the union said it is also demanding the new contract provide more affordable health insurance options and limit the hospital’s use of temporary nurses.

The union wants a 3 percent raise for the first six months of the contract and a 4 percent raise for the next 12 months. The hospital isn’t offering any new across-the-board raise, but notes nurses already receive a 5 percent step increase on the yearly anniversary of their hiring until they reach two decades of service.

Brigham nurses start at $86,700 annually, and those with two decades of service earn at least $220,000, according to documents shared by MGB. Nearly one-third of the local, or 1,322 nurses, earn that maximum amount, MGB said, making them among the highest paid in the state.

Over the past five fiscal years, Brigham said Friday, nurses’ base wages have increased by an average of 55 percent.

Brigham nurses say the across-the-board raise is crucial because of inflation, the high cost of living in Greater Boston, and to retain highly trained employees.

“We have repeatedly tried to get MGB to negotiate and avoid a second strike, but the billionaires who run MGB have shut the door on us,” Kelly Morgan, a Brigham labor and delivery charge nurse who heads the union bargaining unit, said last week.

MGB contends the union’s wage demands are financially unsustainable.

A long-lasting strike could wreak havoc at Brigham, a Harvard-affiliated teaching hospital that attracts patients from around the world, and force MGB to shell out millions of dollars for fill-in nurses.

Marin Wolf of the Globe staff contributed to this report.

Jonathan Saltzman can be reached at jonathan.saltzman@globe.com.