WASHINGTON — Four years after Abdul Qadeer Khan, the leader of the world's largest black market in nuclear technology, was put under house arrest and his operation declared shattered, international inspectors and Western officials are confronting a new mystery, this time over who may have received blueprints for a sophisticated and compact nuclear weapon found on his network's computers.

Working in secret for two years, investigators have tracked the digitized blueprints to Khan computers in Switzerland, Dubai, Malaysia and Thailand. The blueprints are rapidly reproducible for creating a weapon that is relatively small and easy to hide, making it potentially attractive to terrorists.