Emissions of a banned, ozone-depleting chemical are on the rise, a group of scientists reported, 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.

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