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 for what’s 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 source.

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

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

But for now, 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 east Asia.

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 the U.S. National Oceanic and Atmospheric Administration with help from scientists in the Netherlands and the United Kingdom. Their results were published in the journal Nature.

The scientists said they considered a range of alternative explanations, 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, they said.