How did Thresh know that Katniss befriended Rue in Hunger Games?

Question

In The Hunger Games, Katniss is attacked by Clove, but saved at the last moment when Thresh kills Clove. Thresh is then in a position to kill Katniss, too, but says (quoting from memory):

Just this once, Twelve. For Rue.

Then runs back into the woods.

How did Thresh know that Katniss had befriended Rue?

Answer 1

If I recall, as Clove has Katniss pinned and is about to kill her, the two women share some dialog about Rue. Thresh hears this and kills Clove.

So, to reword it, Clove gloated about the death of Rue and Thresh overheard and killed her in his anger, but also saved Katniss for his gratitude towards her for taking care of her since he and Rue are both from District 11.

Answer 2

In addition to overhearing the conversation between Katniss and Clove in the arena, Thresh may have also noticed that Rue had been "shadowing" Katniss during the training sessions. This may lend some credibility to Katniss' story.

Also, if I remember both correctly, I think there was slightly more dialogue in the book between Thresh and Katniss during the scene in the arena. He asks her if what he overheard was true, who killed Rue, if Katniss killed the other tribute, etc.