In February 2014, Facebook purchased the messaging service WhatsApp for $19 billion. The acquisition price was staggering for an app that made little money and was largely popular outside the United States.

Now, newly published confidential Facebook emails and charts show exactly why CEO Mark Zuckerberg spent a small fortune for the messaging app. For months, the company had been tracking WhatsApp obsessively using Onavo, a VPN and data analytics app, whose data showed that the messaging app was not just a rising competitor, but a potential Facebook killer.

The “highly confidential” charts — part of a trove of documents released today by the United Kingdom’s digital, culture, media, and sport (DCMS) parliamentary committee — show WhatsApp was a growing juggernaut in the messaging app wars of the early 2010s.

The previously sealed documents, obtained by the DCMS committee from an app developer who is currently suing Facebook in a California court, shed light on a variety of previously unknown-to-the-public practices used by the social network between 2012 and 2015, including providing favorable, whitelisted access to user data to large app partners. The emails and papers, obtained by the plaintiff in legal discovery, also show that Zuckerberg once considered charging developers for access to user data, though that policy was never implemented.

While Facebook (and then newly Facebook-acquired Instagram) topped Onavo’s US mobile apps charts for iPhone, WhatsApp was growing steadily in its percentage of market reach, beating out popular mobile apps including Tumblr, Foursquare, Vine, and Google+, while encroaching on Pinterest.