What did Snape know, and When did he know it?

[pippin_999]

No: HPFGUIDX 169373

> Mike:
> Sirius' hatred of Wormtail and his resulting imprisonment because of 
> being bested by Peter is what he used to keep his sanity in Azkaban. 
> This is one area that he didn't need to and doesn't resort to 
> hyperbole, imo. And, yes, we don't know who these unnamed DEs that 
> screamed things out in their sleep were. 

Pippin:
One of them was Crouch Jr. Sirius heard him screaming for his mother.
We know that Sirius was kept heavily guarded, so he probably didn't
get much chance to mingle. He says he  saw Bellatrix as she was being
brought in. I don't think it's too speculative that the prisoners near
him were the Pensieve Four: Crouch Jr, Bella, Rabastan and Rudolphus --
all greatly feared high security prisoners. 

They seem to have known that the real secret keeper was Wormtail
(or someone they knew by that alias, at any rate.) But how many
other Death Eaters knew this? Sirius speculates that one at least does,
and this was why Wormtail needed to stay in hiding. I think he was
quite right about that. 

But it seems extremely unlikely that they would have shared this 
information with Snape, who was stationed at Hogwarts at the
time and so cannot have been spying on the Marauders. 
Probably the only information Snape was given pointed at the 
person Voldemort had picked as his patsy: Sirius Black.

There's canon that Bella and Snape have never been close. She has
no idea where he lives. Snape is close to Narcissa and Lucius,
but they  were not "sent"  (Jo's word) along with the Pensieve 
Four to attack the Longbottoms. I have speculated
before that the real spy was the Sender, and his plan was to 
sting those who could have revealed his true identity. But we'll
soon find out.  

Pippin