The use of civilian software to discuss sensitive military and government matters came to light after a journalist from The Atlantic magazine said he was mistakenly added to a group chat on Signal.

The breach left military and intelligence experts asking the same questions as the public: Why would top U.S. officials use a free messaging app to discuss classified military plans?

Authorities in Paris said on Monday that Durov is being held on questions stemming from an investigation into criminal activity on the app, including the spread of child pornography and facilitating the selling of illegal drugs.