New York State’s attorney general waded into the debate over policing in New York City’s transit system on Monday, saying she was investigating whether the police were discriminating against people of color in enforcing fare evasion on subways and buses.

In a letter to Dermot F. Shea, the police commissioner, the office of the attorney general, Letitia James, requested enforcement data, policies and other information that she said “may shine a light on whether officers have exhibited racial biases or engaged in discriminatory practices.”

“If groups of New Yorkers have been unfairly targeted because of the color of their skin, my office will not hesitate to take legal action,” Ms. James said in a statement accompanying the letter.

Police officials have long denied that racial factors play a driving role in how officers decide whom to ticket or charge with fare evasion on subways and buses.

Devora Kaye, a police spokeswoman, said in response to Ms. James’s announcement that the Police Department’s “transit officers patrol day and night to keep six million daily riders safe and enforce the law fairly and equally without consideration of race or ethnicity.”

In a television interview on NY1, Mayor Bill de Blasio said he supported Ms. James’s inquiry.

“The attorney general’s asking an important question, and we’ll certainly work with her,” Mr. de Blasio said. With regard to the enforcement of fare evasion and policing more broadly, he added, “We want to make sure everything’s fair.”

The civil-rights investigation comes several months after several black and Hispanic officers signed affidavits as part of a discrimination lawsuit in which they claim they were ordered by a commander in Brooklyn to think of white and Asian commuters as so-called soft targets and to instead pursue blacks and Latinos for minor offenses like jumping turnstiles.

The officers’ affidavits, which cover a period from 2011 to 2015, were only made public last month as a crackdown on fare evasion in the subway was becoming a contentious political issue.

Transit authorities have authorized a plan by Gov. Andrew M. Cuomo to add hundreds of additional officers to the subways, fueling a debate about overly aggressive policing and making poverty a crime.

Videos of arrests of young black men and a Latino churro vendor in the subway in recent months went viral online, stirring anger among those who believe the police are too aggressive and prompting protests that included turnstile-jumping.

The Metropolitan Transportation Authority, which operates the city’s subways and buses, has said it is losing millions of dollars because of fare evasion and that it needs the 500 transit police officers proposed by Mr. Cuomo to address the problem. Some elected officials have opposed the move, saying it will worsen the racial disparities in arrests.

“Fare evasion is a $300 million annual problem that should be addressed in a way that does not unjustly target any specific group or community,” Ken Lovett, an authority spokesman said. “We are committed to assisting the attorney general with her inquiry in any way we can.”

From October 2017 to June 2019, during stops when race was recorded, 73 percent of the people who received a ticket for fare evasion and 90 percent of those who were arrested on that charge were black and Hispanic, according to police data.

In the letter sent to Mr. Shea, civil rights investigators from Ms. James’s office asked that the Police Department to turn over documents that would provide a complete picture of fare evasion enforcement since October 2017.

The materials sought include the department’s policies and training for officers, as well as correspondence between the police and the authority on the subject of fare evasion. Ms. James asked the documents be turned over by February 10.

Ms. James’s office also requested data on the race and age of people accused of not paying their fare on a station-by-station basis. The City Council passed a law in 2017 that required the Police Department to post similar information online. The department resisted doing so until a September 2019 legal ruling forced it to comply with some measures.

“It’s just very refreshing that the attorney general is throwing the weight of her office behind getting to the bottom of all of this,” said Councilman Rory I. Lancman, who filed the lawsuit that led to that ruling, “because the N.Y.P.D. has attempted to stymie the Council at every turn, not providing information at the hearings that we have, not complying with the law that we passed, and fighting our lawsuit.”