Emissions of a banned, ozone-depleting chemical are on the rise, a group of scientists reported Wednesday, suggesting someone may be secretly manufacturing the pollutant in violation of an international accord.

Emissions of CFC-11 have climbed 25 percent since 2012, despite the chemical being part of a group of ozone pollutants that were phased out under the 1987 Montreal Protocol.

“I’ve been making these measurements for more than 30 years, and this is the most surprising thing I’ve seen,” said Stephen Montzka, a scientist with the National Oceanic and Atmospheric Administration who led the work. “I was astounded by it, really.”

It’s a distressing result amid what is widely seen as a global environmental success story, in which nations — alarmed by a growing “ozone hole” — collectively took action to phase out chlorofluorocarbons.

The finding seems likely to prompt an international investigation into the mysterious source.

Officially, production of CFC-11 is supposed to be at or near zero — at least, that is what countries have been telling the U.N. body that monitors and enforces the Montreal Protocol. But with emissions on the rise, scientists suspect someone is making the chemical in defiance of the ban.

“Somebody’s cheating,” Durwood Zaelke, founder of the Institute for Governance and Sustainable Development and an expert on the Montreal Protocol, said in a comment on the new research. “There’s some slight possibility there’s an unintentional release, but … they make it clear there’s strong evidence this is actually being produced.”

The scientists don’t know exactly who, or where, that person would be. A U.S. observatory in Hawaii found CFC-11 mixed in with other gases that were characteristic of a source coming from somewhere in eastern Asia, but scientists could not narrow the area down any further.

Zaelke said he was surprised by the findings, not just because the chemical has long been banned but also because alternatives exist, making it hard to imagine what the market for CFC-11 today would be.

The research was led by U.S. researchers with NOAA, with help from scientists in the Netherlands and the United Kingdom. Their results were published in the journal Nature.

There is a small chance that there is a more innocent explanation for the rise in CFC-11 emissions, the scientists say.

They considered a range of alternative explanations for the growth, such as a change in atmospheric patterns that gradually remove CFC gases in the stratosphere, an increase in the rate of demolition of buildings containing old residues of CFC-11, or accidental production.

But they concluded these sources could not explain the increase, which they calculated at about 13 billion grams per year in recent years. Rather, the evidence “strongly suggests” a new source of emissions, the scientists wrote.

“These considerations suggest that the increased CFC-11 emissions arise from new production not reported to [the U.N. Environment Program’s] Ozone Secretariat, which is inconsistent with the agreed phase-out of CFC production in the Montreal Protocol by 2010,” the researchers wrote.

CFC-11, used primarily for foams, can last up to 50 years in the atmosphere once it is released. It is destroyed only in the stratosphere, some nine to 18 miles above the planet’s surface, where the resulting chlorine molecules engage in a string of ozone-destroying chemical reactions. That loss of ozone, in turn, weakens our protection from ultraviolet radiation at Earth’s surface.

The chemical is also a powerful greenhouse gas that contributes to global warming.

The paper’s findings are “environmentally and politically quite serious,” Robert Watson, a former NASA scientist who organized bracing flights high into the Antarctic stratosphere to study ozone depletion in the 1980s, said in an emailed statement.

“It is not clear why any country would want to start to produce, and inadvertently release, CFC-11, when cost-effective substitutes have been available for a long while,” Watson continued. “It is therefore imperative that this finding be discussed at the next Ministerial meeting of Governments given recovery of the ozone layer is dependent on all countries complying with the Montreal Protocol (and its adjustments and amendments) with emissions globally dropping to zero.”

Watson suggested that aircraft flights might be necessary to better identify the source of the emissions.

Keith Weller, a spokesman for the U.N. Environment Program, which administers the Montreal Protocol, said the findings would have to be verified by the scientific panel to the protocol and then would be put before the treaty’s member countries.

“If these emissions continue unabated, they have the potential to slow down the recovery of the ozone layer,” Weller said in a statement. “It is therefore, critical that we take stock of this science, identify the causes of these emissions and take necessary action.”

Unreported production of CFC-11 outside certain specific carve-out purposes in the treaty would be a “violation of international law,” Weller confirmed, though he said that the protocol is “non-punitive” and the remedy would probably involve a negotiation with the offending party or country.

But Zaelke said the finding could promote tougher action.

“This treaty cannot afford not to follow its tradition and keep its compliance record,” he said.

“They’re going to find the culprits,” Zaelke said. “This insults everybody who’s worked on this for the last 30 years. That’s a tough group of people.”

The ozone layer is slowly recovering, and ozone-depleting substances are still declining. But the apparent increase in emissions of CFC-11 has slowed the rate of decrease by about 22 percent, the scientists found. This, in turn, will delay the ozone layer’s recovery and, in the meantime, leave it more vulnerable to other threats.

“Knowing how much time and effort and resources have gone into healing the ozone layer and to see this is a shocker, frankly,” Montzka said.