This is amazing. This images were released yesterday by a famous Globo Actor, Rodrigo Andrade, an actor from one of the biggest TV Medias in Brazil.
The UFO was sighted on 12 October of 2013. The images weren't release back then because the witness were afraid of repercussion.
The reason they were unable to spot it sooner was simple: The object wasn't appearing in the radar. After the pilots contacted the tower of command for info on the mysterious object, they awnsered back that the plane was alone and there was no object in the radar. Which is strange because you can clearly see in the images an UFO.
The object followed the plane for around 25 minutes, until it suddenly vanished. The co-pilot and the pilot describe it as looking like a colored papper.
Here's the video: