Why is BlackBerry taking legal actions against Facebook Messenger?

Blackberry is suing Facebook Messenger. But why? There's all sort of rumors flying around but according to court doccuments which have been leaked, it's because Blackberry claimes that its messenger was stolen by Facebook.

It's the technology behind the app that has Blackberry sure they were infringed upon. Blackberry was a pioneer of the smart phone and secure technology features but got a little behind the times when other smart phones became available. Now, they are in a legal battle with social media. But, to make matters more dramatic and more complicated as well, Facebook is counter-suing Blackberry. They claimed that it was they who were infringed upon. Who's right and who is wrong? It may take a judge to figure that one out.