Some of the American technicians have had direct access to the nuclear weapons themselves, these sources said.
In any case, Pakistan's nukes are currently secure, in the opinion of several former and serving U.S. officials. "They are for now," said one.
The concern over Pakistan's arsenal extends back in time, before the Sept. 11, 2001 attacks. As early as 2000, the Clinton administration created a joint commission, a 'liaison' group, consisting of top American and Pakistani scientists. The purpose of this group was to help the Pakistanis create command and control codes for its nuclear weapons that would be unbreakable. One former senior U.S. intelligence source told me that in the course of such work, America gained "a pretty full knowledge" of Pakistan's command and control system.
The United States then used Special Forces 'snatch teams' to kidnap Pakistani scientists who were peddling Pakistan's nuclear technology or knowledge of it to undesirables. For example, a group of such scientists abruptly disappeared while traveling in Burma, these sources said.
In addition, the kidnappings disrupted an alleged 200 links between the Pakistani nuclear community and terrorists with ties to al-Qaida, they said. Other Pakistanis sympathetic to al-Qaida such as Sultan Bashiruddin, a much-decorated scientist in Pakistan's nuclear community, were arrested and interrogated.
The fact was that even before 9/11, U.S. intelligence had thoroughly infiltrated the nuclear smuggling ring of Pakistan's lead nuclear scientist, A.Q. Khan, without disclosing this to the government of Pakistan. The penetration proved a chief factor in Libya's abandoning its own nuclear program and why Iran, another Pakistan client, disclosed its own activities to the International Atomic Energy Agency, the Vienna watchdog group.
After the 9/11 attacks, American aid to Pakistan to safeguard and secure its arsenal was drastically stepped up, with the George W. Bush administration using the proposed $3 billion U.S. aid package that included F-16s and all sorts of advanced hardware, as a bludgeon. Under U.S. pressure, within two days of the attacks, Pakistan's military began to secretly relocate critical nuclear weapons components to six new secret locations, U.S. sources said. Warheads and delivery systems, which were already being kept separated, were put even more widely apart, and additional surveillance was put on Pakistan's nuclear labs and their personnel, they said.
Additional steps were also taken to separate fissile material from the labs or the weapons themselves, they said.
More U.S. 'technical advisory' teams, many staffed by Defense Intelligence Agency or Energy Dept. intelligence officials, began to appear in Pakistan along with warning and assessment equipment.
Communications systems between Pakistani nuclear commanders and nuclear storage sites were reviewed and modernized, and certain key nodes were, at some point, on a U.S. target list, sources said.
Thanks to U.S. technical means, the United States became aware of defects and miscommunication between Pakistani military centers of command during atomic tests which helped U.S. analysts to grasp facets of Islamabad's command and control areas that were of dubious reliability.
Following 9/11, when U.S. advisors persuaded Pakistani scientists to adopt some key features that add security to U.S. nuclear command procedures, tension rose over whether to install Permission Action Links (PALs), an electronic lock that renders a weapon null and void until political commanders relinquish control of the special codes that allow the weapon to be turned on, several sources said. In addition, the weapons could not be used without employing a dual-key system, meaning that a single rogue commander could not initiate their use.
In brief, the PALs would prevent the unauthorized use of a nuclear weapon by an aberrant member of the military, and they would prevent use of such a weapon by terrorists, and therefore are important, U.S. officials said.
Yet disputes arose immediately. There were legal implications about sharing such sensitive military technology with a foreign power, and some senior U.S. officials balked at using the PALs, thinking they would give the Pakistanis too much insight into America's own nuclear war fighting system. "The Paks are smart. What they can see and examine, they can re-engineer," said one.
For their part, the Pakistanis feared that American scientists would insert a 'dead switch' into the PALs, which would freeze the weapons if someone attempted their use, similar to being able to stall a stolen car from a remote position.
There is some ground for Pakistani misgivings. For years, U.S. intelligence has infiltrated the front companies used by Iran to acquire nuclear weapons technology from the West, especially Europe. Many of these companies were originally part of the Pakistani network set up by A.Q. Khan that procured both components and information for North Korea, Libya and the like. Many are engineering consulting firms, U.S. officials said.
An atomic bomb requires enriched uranium, and to enrich uranium, machines called centrifuges are required – rapidly spinning tubes that are used to separate and concentrate isotopes in gasified uranium. Spinning at several thousand revolutions per minute, they rest on superb bearings, in perfect balance, in a vacuum, linked by pipes to thousands of other spinning units. When the process works, the gas ends up in a solid form, but any minute defect, and the product is decisively marred.
The same is true of the other equipment required: tools, magnets, exotic steel, vacuum pumps, ball bearings and instruments of all kinds, all must be perfect.
Iran uses front companies, fake end-user certificates and third-country destinations to disguise the true purpose, but according to one former senior CIA official, "We have infiltrated such companies and have been able to insert flaws into the technology that we can exploit. It goes along the line of our selling computers that have trap doors into which U.S. technicians can enter to manipulate the machine."
During the Kosovo war, NSA systems were able to make false insertions into the workings of Serb air defense radars, rendering them inoperable.
Other Iranian targets include electronic circuits, electromagnetic machines called caultrons, industrial circuits, power supplies, and compressors for window mounted air conditioners.
"The point is that when they push the button, the stuff won't work," the former senior official said.
He and others said that the operation "is fairly long-standing" and successful.
Pakistan is said to have between 25-40 strategic nuclear weapons, and Jane's Defense Weekly says Pakistan also has about 60 short-to-medium range missiles and 34 F-16s capable of delivering an atomic warhead. Islamabad exploded its first weapon in 1998.