The Exxon inquiry might expand further to encompass other oil companies, according to the people with knowledge of the case, though no additional subpoenas have been issued to date.

The people spoke on the condition of anonymity, saying they were not authorized to speak publicly about investigations that could produce civil or criminal charges. The Martin Act, a New York state law, confers on the attorney general broad powers to investigate financial fraud.

To date, lawsuits trying to hold fuel companies accountable for damage they are causing to the climate have failed in the courts, but most of those have been pursued by private plaintiffs.

Attorneys general for other states could join in Mr. Schneiderman’s efforts, bringing far greater investigative and legal resources to bear on the issue. Some experts see the potential for a legal assault on fossil fuel companies similar to the lawsuits against tobacco companies in recent decades, which cost those companies tens of billions of dollars in penalties.

“This could open up years of litigation and settlements in the same way that tobacco litigation did, also spearheaded by attorneys general,” said Brandon L. Garrett, a professor at the University of Virginia School of Law. “In some ways, the theory is similar — that the public was misled about something dangerous to health. Whether the same smoking guns will emerge, we don’t know yet.”

In the 1950s and ’60s, tobacco companies financed internal research showing tobacco to be harmful and addictive, but mounted a public campaign that said otherwise and helped fund scientific research later shown to be dubious. In 2006, the companies were found guilty of “a massive 50-year scheme to defraud the public.”

The history at Exxon Mobil appears to differ, in that the company published extensive research over decades that largely lined up with mainstream climatology. Thus, any potential fraud prosecution might depend on exactly how big a role company executives can be shown to have played in directing campaigns of climate denial, usually by libertarian-leaning political groups.

For several years, advocacy groups with expertise in financial analysis have been warning that fossil fuel companies might be overvalued in the stock market, since the need to limit climate change might require that much of their coal, oil and natural gas be left in the ground.

The people with knowledge of the case said the attorney general’s investigation of Exxon Mobil began a year ago, focusing initially on what the company had told investors about the risks that climate change might pose to its business.

News reporting in the last eight months added impetus to the investigation, they said. In February, several news organizations, including The New York Times, reported that a Smithsonian researcher who had published papers questioning established climate science, Wei-Hock Soon, had received extensive funds from fossil fuel companies, including Exxon Mobil, without disclosing them. That struck some experts as similar to the activities of tobacco companies.

More recently, Inside Climate News and The Los Angeles Times have reported that Exxon Mobil was well aware of the risks of climate change from its own scientific research, and used that research in its long-term planning for activities like drilling in the Arctic, even as it funded groups from the 1990s to the mid-2000s that denied serious climate risks.