An investigation into Allen Weisselberg, the Trump Organization’s finance chief, appears to be heading into its final stages as prosecutors increase pressure on him.

The Manhattan district attorney’s office appears to have entered the final stages of a criminal tax investigation into Donald J. Trump’s long-serving chief financial officer, Allen H. Weisselberg, setting up the possibility he could face charges this summer, according to people with knowledge of the matter.

In recent weeks, a grand jury has been hearing evidence about Mr. Weisselberg, who is facing intense scrutiny from prosecutors as they seek his cooperation with a broader investigation into Mr. Trump and the Trump Organization, the people with knowledge of the matter said. The prosecutors have obtained Mr. Weisselberg’s personal tax returns, the people said, providing the fullest picture yet of his finances.

Even as the investigation has heated up, it remains unclear whether the prosecutors will seek an indictment of Mr. Weisselberg, which would mark the first criminal charges stemming from the long-running financial fraud investigation into Mr. Trump and his family company.

The investigation into Mr. Weisselberg focuses partly on whether he failed to pay taxes on valuable benefits that Mr. Trump provided him and his family over the years, including apartments and leased cars as well as tens of thousands of dollars in private school tuition for at least one of his grandchildren. In general, those types of benefits are taxable, although there are some exceptions, and the rules can be murky.

For months, prosecutors working for District Attorney Cyrus R. Vance Jr., a Democrat, have sought to pressure Mr. Weisselberg into cooperating with their investigation into Mr. Trump, and any deal could turn the trusted executive into a star witness against the former president. For now, Mr. Weisselberg appears to have rebuffed Mr. Vance’s office and continues to work at the Trump Organization.

The district attorney’s office recently questioned Mr. Weisselberg’s top lieutenant, Jeffrey S. McConney, before a special grand jury hearing evidence in the Trump inquiry, people with knowledge of the matter have said. The testimony was the first sign that the grand jury was hearing evidence about Mr. Weisselberg.

When hoping to turn an insider into a cooperating witness, prosecutors often seek leverage over the person, and then typically offer leniency in exchange for testimony or assistance.

The Trumps have long been able to count on Mr. Weisselberg’s fealty. After beginning his career working for Mr. Trump’s father, Mr. Weisselberg has served as the Trump Organization’s financial gatekeeper for more than two decades.

Even if Mr. Weisselberg chooses not to assist the investigation into his boss, charges against him could portend trouble for Mr. Trump, signaling that the prosecutors have identified what they believe is misconduct at his family business.

As part of the investigation into the fringe benefits Mr. Trump provided, Mr. Vance’s prosecutors have sought records for Mercedes-Benz cars leased for Mr. Weisselberg, his wife and other Trump Organization employees over the course of more than a decade, according to a person with knowledge of the matter.

The full scope of the investigation into Mr. Weisselberg, including whether prosecutors are considering other charges against him separate from the fringe benefits, could not be determined. It is rare for prosecutors to build a criminal case solely around a failure to pay taxes on fringe benefits.

Mr. Vance’s wider investigation into the Trump Organization has included scrutiny of whether Mr. Trump and the company manipulated property values to obtain certain loans and tax benefits, among other potential financial crimes.

Mr. Trump has called the investigation by the Manhattan district attorney’s office a politically motivated fishing expedition. 
Travis Dove for The New York Times

A lawyer for Mr. Weisselberg, Mary E. Mulligan, declined to comment, as did a Trump Organization representative. Mr. Trump, a Republican, has long lashed out at the investigation, calling it a “continuation of the greatest political Witch Hunt in the history of our country.” A spokesman for Mr. Vance, who has served three terms but is not running for re-election this year, declined to comment.

Before recently convening the special grand jury, Mr. Vance’s office was already using other grand juries to issue subpoenas for documents and hear some testimony. The new panel is expected to hear from a wide range of witnesses in the coming months and, if prosecutors present charges, could vote on indictments. Still, there is no indication that the inquiry into Mr. Trump has reached that advanced stage, or even that prosecutors have decided to seek charges against the former president or his company.

The New York State attorney general, Letitia James, who had been conducting a civil examination of some of the same issues that the district attorney’s criminal investigation is examining, has joined Mr. Vance’s inquiry. She, too, has obtained Mr. Weisselberg’s tax returns, people with knowledge of the matter said.

While pursuing Mr. Weisselberg’s cooperation, Mr. Vance’s prosecutors appeared to be building a picture of his financial life, securing earlier this year not only his tax returns and the underlying documents but also his personal bank records. They also asked the Trump Organization to turn over documents related to any benefits Mr. Trump or the company may have provided to other employees.

The investigation has led the prosecutors to subpoena the records of Columbia Grammar and Preparatory School, an Upper West Side private school, seeking information about tuition payments Mr. Trump made on behalf of at least one of Mr. Weisselberg’s grandchildren.

Prosecutors additionally questioned Mr. Weisselberg’s former daughter-in-law, Jennifer Weisselberg, who is in the midst of a contentious custody battle with her ex-husband, Barry. Her lawyer, Duncan P. Levin, has said she has been interviewed six times and is cooperating with the investigation.

Ms. Weisselberg has said that prosecutors had asked her about the tuition payments as well as gifts Barry Weisselberg received from Mr. Trump, including an apartment on Central Park South and several cars that were leased for him. Barry Weisselberg manages the Trump Wollman Rink in Central Park.

The prosecutors have also focused on whether Mr. Trump provided Allen Weisselberg with a Manhattan apartment.

Mr. Weisselberg’s lawyers could argue that some of the benefits were not taxable, or that Mr. Weisselberg did not know he needed to pay taxes on them. The rules around tuition, for example, are somewhat open to interpretation.

If prosecutors eventually seek charges against Mr. Weisselberg based on the fringe benefits, depending on what the evidence shows, they could choose from several potential crimes, including grand larceny, scheme to defraud or tax fraud, experts said.

To prove scheme to defraud, Mr. Vance’s prosecutors would need to show that Mr. Weisselberg engaged in a “systematic ongoing course of conduct with intent to defraud.” To prove he committed tax fraud, they would have to show that he willfully failed to pay taxes on the benefits.

With tax fraud, the fallout for Mr. Weisselberg would be steeper than under the scheme to defraud charge: Failing to pay more than $10,000 in taxes in a single year can be punishable by up to seven years in prison, while the penalty for scheming to defraud is a maximum of four years.

“Those dollar amounts could make it relatively easy for the Manhattan district attorney to make a criminal case,” said Cono R. Namorato, a lawyer at Caplin and Drysdale and a former senior official at the Justice Department’s tax division.

Still, Mr. Namorato and other tax lawyers said that it would be unusual to bring such a case on failure to pay taxes on fringe benefits alone; the lawyers could think of no other recent example.

Mr. Weisselberg has kept a low profile during his long tenure at the company, but his name surfaced three years ago in connection with a federal investigation into hush money paid during the 2016 presidential campaign to two women who said they had affairs with Mr. Trump. Michael D. Cohen, Mr. Trump’s longtime fixer who helped arrange the payments to the women, has said Mr. Weisselberg was involved as well.

Mr. Cohen, who pleaded guilty to federal charges in 2018, is now cooperating with Mr. Vance’s investigation.

Kate Christobek contributed reporting.