WASHINGTON - An infamous atomic smuggler may have had blueprints for a compact, sophisticated nuclear warhead, and that could mean that the world's proliferation problem is even worse than many experts had thought.

US officials have long declared the nuclear technology ring run by Pakistani scientist Abdul Qadeer Khan to be shattered. But revelations that a digitized bomb design turned up on the computer of an associate of Mr. Khan's show that US and UN investigators may not yet know everything Khan did, despite the fact that he has been under house arrest in Pakistan for years.