We've covered the problems at Ambac and its fellow monolines before -
Crisis point
Monolines show no appetite for Buffett
US regulators reveal plan to split monolines
Dragged down

Ambac's also made a $1 billion loss provision for its exposure to mortgage-backed securities, and there are now suggestions again that it could lose its vital AAA credit rating. Shares are down 45%. MBIA's fallen almost as far, and it seems to have doomed SCA's latest attempt to get its share price back above the $1 mark.

And there's yet more. According to the conference call yesterday, Ambac's now investigating several of its deals, looking for - well, it's not quite clear, but it sounds like they're looking for fraud, and it sounds like they're starting to find it.
Transcript (via Calculated Risk):

David Wallis, Chief Risk Officer: ...it is very striking how concentrated, how very concentrated, some of the poor performers are, and that gives rise to all sorts of obvious questions.

I mentioned that we have diagnostic and forensic people working on some of these deals. We are beginning to see stuff back from that. The diagnostic is basically running tapes looking at delinquencies and trying to figure out given what you now know and what you knew then, would you have expected that delinquency or not? And if the answer is not, well, that is interesting.

So, in other words, you have an incredibly low FICO within a pool, and it is delinquent. Well, maybe you expected that. But if it is incredibly high and the LTV was incredibly low, then maybe you would not expect that.

So then what you do is, you take an adverse sample, so you run the tape through a program, take an adverse sample, i.e. looking for the suspicious ones, and then what you do is you go look at the files. That is a very difficult long process, but you look in the files. You look at the transcripts of servicing records, and you see what you see. And all I will say is that there is some pretty amazing stuff to see.