New York's attorney general has accused ExxonMobil of misleading investors by using two sets of numbers in its greenhouse gas accounting—one shown to investors, and a "secret" set used internally by the company.

In a document filed in a New York court on Friday, Attorney General Eric Schneiderman says that Rex Tillerson, who was then the chief executive of Exxon and is now the U.S. secretary of state, approved of what "may be a sham."

Schneiderman spelled out the evidence for the first time from documents and testimony already turned over by Exxon, and demanded more information under additional subpoenas in a probe that has been going on since 2015.

The details of the allegation provide the most substantive evidence yet that the energy company may have misrepresented the financial risks posed to its business by climate change.

The filing says that Tillerson was "specifically informed of, and approved of" the discrepancy.

The assertion revolves around a so-called "proxy cost" for carbon dioxide emissions, an estimate of the penalties governments may impose in the future on greenhouse gases.

In the absence of a price on carbon, companies can use proxy costs to assess investments. Schneiderman described evidences showing that while Exxon had one set of estimates that it told investors about, it actually applied a lower proxy cost when it made investment decisions, with the effect of making its investments look more attractive.

The attorney general's office "has uncovered significant evidence of potential materially false and misleading statements by Exxon about its application of a proxy cost" to its decisions, the filing says, "suggesting that the exercise described to investors may be a sham."

The attorneys general of New York and Massachusetts and the U.S. Securities and Exchange Commission have each been investigating Exxon's accounting practices, focusing on whether the company misled investors about the risks of climate change.

On Wednesday, the company's shareholders passed a landmark resolution, with 62 percent of the vote, requiring Exxon to report on its climate risk.

The new allegations appear to focus on the company's investments in Canada's tar sands, or oil sands, among the most expensive and greenhouse gas-intensive sources of oil.

Exxon is the largest foreign operator in the tar sands. Over the past few decades, oil sands have grown to represent 12 percent of its liquid reserves, and Exxon has remained committed there even as other major international companies, including Shell and ConocoPhillips, have sold much or all of their oil sands portfolios over the past year. In February, Exxon announced that it would remove from its books 3.5 billion barrels of tar sands reserves, acknowledging that they were not profitable at current oil prices. The move was only an accounting measure, however, and the company said it would continue to produce oil there anyway and would add back the reserves if oil prices rebound.

The court filing says that since 2007, Exxon has told investors that it included a proxy cost in its calculations when evaluating whether to invest in new oil and gas projects. Tar sands projects present a particular challenge, because they generally require initial investments in the billions of dollars and then are expected to operate for decades, over which time climate regulations presumably will tighten.

In 2014, the company said in a public report that the proxy cost it used ran as high as $80 per ton when the company was looking forward to 2040. The report also said the company was "confident that none of our hydrocarbon reserves are now or will become 'stranded'."

But according to Schneiderman's filing, Exxon actually used a lower proxy cost to evaluate projects.

The document cites an exchange between an employee of Imperial Oil, Exxon's Canadian affiliate, and the parent company, revealed in documents uncovered by the New York attorney general's investigation. The employee was evaluating a project in the tar sands, and Exxon apparently told him not to apply the costs contained in the company's corporate directives, but instead to use the much lower levels of Alberta's provincial carbon tax.

The employee pushed back, but eventually relented, according to the document filed in court.

Had the company applied the higher, publicly stated cost, the filing says, "at least one substantial oil sands project may have projected a financial loss, rather than a profit, over the course of the project's original timeline."

It's unclear whether this refers to Exxon's Kearl project, its newest, which was the subject of the reserves reduction announced in February.

Exxon did not respond to requests for comment.

The filing says that Exxon ended the practice of using two separate proxy costs in 2014, but that the company never revealed the discrepancy and "may still be in the midst of perpetrating an ongoing fraudulent scheme on investors and the public."

Schneiderman's office has issued new subpoenas requesting documents and testimony from several witnesses. Exxon filed a motion to quash some of those subpoenas, and Friday's filing was a response to that motion.

The filing makes clear that Schneiderman's office believes company executives were aware that they were misleading investors.

"Exxon actually recognized that its secret, internal figures understated the degree to which Exxon was taking into account the risks of climate change regulations," the filing says, "and thus, were not as conservative as its representations to investors suggested when applied to the vast majority of projects."