Suspects also are under indictment in Nebraska.

In a public service announcement and a separate, broader bulletin, the bureau says the problem is real, and the losses to businesses and other organizations are "significant."

The "SilverTerrier" group has evolved into a mature hacking threat over the past five years, researchers say.

There's so much available in cybercrime forums that scammers don't need to be very creative, the X-Force team found after examining billions of incidents.

Phishing attempts come with a Microsoft Word document that activates a strain of malicious software, AZORult, which allows attackers to make off with sensitive data.

Palo Alto Networks' Unit 42 found more activity that appears to be from a group that so far has evaded attribution, but has shown certain habits in its spearphishing campaigns.