[shoes_for_thee]

Beats me. I can speculate a bit:

Perhaps professionals would be less likely to remove casings because they do not expect to ever go to trial, or know that the gun they used can not be connected to them (because perhaps it was issued to them for this job), are not local to the area or do not intend to remain in the US after the job.

Only 100 casings were found, which does not strike me as very much... Basically just three people dumping a mag each + a smidge extra.

It is interesting that the article mentions only finding 7.62x39. This kinda says non-NATO to me, or at least the shooters want us to think "ruskies!"

[sio8ohPi]

I wouldn't read anything into the choice of cartridge. Even assuming the shooters were foreign, there's not much reason for them to import the guns, rather than just sourcing them locally, and 7.62 is common in California.