On its face, the decision to release Mr. Abdul Qadeer Khan from house arrest in Pakistan is a slap in the face of international opinion and a blow to efforts to halt the proliferation of nuclear weapons. According to Pakistan's Foreign Ministry, the work of the world's greatest proliferator "is a closed chapter." Were that the truth. Much remains unknown about the A.Q. Khan network and, without that knowledge, it will be impossible to ensure that the technology, materials and knowhow essential to the spread of nuclear weapons remain restricted.

Mr. Khan is a nuclear scientist and engineer, revered in Pakistan as the father of that nation's nuclear bomb program. To help his country keep pace with India, its chief rival and neighbor, he developed a blueprint for a bomb — some say with Chinese help — and a network of suppliers around the globe to produce the components of a nuclear weapon. After Pakistan exploded a nuclear device in 1998, he was lionized for his work and given almost limitless freedom to expand his efforts. Convinced that the global nonproliferation regime was a form of nuclear apartheid, Mr. Khan was eager to serve customers around the world, and did business with governments in Libya, Iran, Syria and North Korea, among others.

Mr. Khan's enterprises ended in 2003, when Libya decided to come clean about its nuclear ambitions — prodded by the seizure of a shipment of imported nuclear materials, which exposed its program — and Mr. Khan's role in their realization. Pakistan arrested Mr. Khan, who then made a public apology for his actions, taking full responsibility. He was granted a pardon by then President Pervez Musharraf, put under house arrest and shielded from inquiry by other countries about his activities.