Littlefinger tells Catelyn Stark that the dagger used in the attempt on Bran's life in season one was Tyrion's dagger, but it was actually Littlefinger's dagger. Arya mentions this in Littlefinger's death scene, which is something the show hasn't really explored yet.
In the books, Littlefinger lost the dagger in a bet with Robert Baratheon. Tyrion and Jaime realize that someone from the royal party that came to Winterfell must have stolen it, and they eventually deduce it was Joffrey. He stole the dagger and hired the assassin to kill Brandon.
The show omitted this entire plotline after Tyrion's trial by combat in the Eyrie, and it seems as though now the writers are saying it was Littlefinger who ordered the assassination. It's a bit unclear because this season has moved at breakneck speed, so there was no other explanation offered.
But at the very least, Littlefinger frames Tyrion for Bran's assassination. That much we know for sure.
