The thieves would be shielded by confidentiality though, so there is no way investigators would be able to track down the thieves without conducting their own investigation. At this point, it seems the FBI is attributing the theft to blind luck on the part of the thieves, and not any inside information as was speculated. The FBI's Newark spokesman, Steve Siegal, says in the NYT,
This time of year, close to Christmas, they probably thought they’d found a truck filled with PlayStations and broke in and started looking for the biggest-looking box. Basically, it’s a target-of-opportunity typical New Jersey cargo theft. There are literally predators — for lack of a better word — who when they see a tractor-trailer or a cargo vehicle parked for any length of time start snooping around.
If anything, that makes the delivery company in charge of transporting the work look even sillier. It's a sad state of affairs when ps2's are harder to steal then a work of art.
I do not anticipate any charges being filed in this case, and the resolution of this mirrors the recovery of a Peruvian gold headdress authorities recovered in London in August. Investigators want to reward thieves who quickly return objects in this way. One of the best shots investigators may have at recovery is if thieves anonymously return stolen objects. Because the objects are so valuable, their safe return is the highest priority. This Goya, like the Peruvian treasure, has a very small potential market. The risk of an arrest pales in comparison with the proceeds of a potential sale, because no reputable buyer would be willing to take on stolen property like this.