Summary Box: Few do most China-based data theft

SMALL NUMBERS: U.S. cybersecurity analysts and experts say that as few as 12 Chinese groups, largely backed or directed by the government there, commit the bulk of the China-based cyberattacks stealing critical data from U.S. companies and government agencies.

HOW THEY KNOW: The aggressive but stealthy attacks often carry distinct signatures allowing U.S. officials to link them to certain hacker teams.

THE PROBLEM: It is largely impossible for the U.S. to prosecute hackers in China, since it requires reciprocal agreements between the two countries, and it is always difficult to provide ironclad proof that the hacking came from specific people.