International investigators are concerned detailed information on how to build an advanced compact nuclear weapon may be circulating on the black market. The digital blueprints have been tied to Pakistani nuclear scientist AQ Khan. Khan’s illicit nuclear network was broken up in 2004, but news of the digital blueprints only came to light in recent weeks. Investigators remain unsure who received the sensitive information or how many copies of the electronic blueprints are circulating. Khan remains under house arrest in Pakistan, but Pakistani officials have refused to allow international nuclear inspectors to interview him.