Really hard to say if they got the private key, but it is really weird if they try to hack back the hackers?
Servers doesn't contained private key, and I believed that the hackers will keep in somewhere safe. So it is really mind boggling, if governments has the ability to track and seized the ransom, then by all means they can get to anyone.
Anyhow, this is clearly an cyber war now against those group of hackers who is targeting, anything, from universities to hospitals to private companies to demand huge amount of money in bitcoins.
I think it is way more likely that somehow they got the hackers and someone confessed and told them how they could access the funds in order to get a reduction of time they will have to serve at jail, while it is obvious governments have the latest and best tools for hacking it is way more simple to appeal to the nature of the people, after all if they can offer a deal to one of the hackers and still put the rest at jail then recovering the funds should be easy, however if the hackers are still free then it is going to be interesting to know how they achieved this.