Check this out: While the United States gives Pakistan $700 million a year in aid, Islamabad still won't give us access to the former CEO of Pakistan's nuclear Walmart, A.Q. Khan - the only outsider with insider knowledge of Tehran's nuclear program.

 According to the Pakistani government, Khan (under house arrest since 2004) and 12 of his cronies (recently acquitted by Pakistani courts) are "off-limits" - and the case of the most egregious example of nuclear proliferation ever is closed.

While the world frets over possible timelines for an Iranian nuclear-weapons breakout, many critical questions about Khan's network still exist. The United States, European Union and United Nations must press Pakistan to fully reveal Khan's involvement with Iran - and others.

What makes Khan so critical to understanding Persia's nuclear puzzle? By all accounts, he's the only person outside of Iran capable of providing some of the missing pieces on the plans and capabilities of Iran's largely clandestine nuclear program.

While lining his pockets with over $100 million, Khan (the father of the Pakistani bomb), passed critical nuclear know-how to Tehran, helping it develop the uranium-enrichment capability needed both to produce nuclear-reactor fuel and to make nuclear weapons.

Pakistan's Dr. Strangelove not only passed Tehran technical engineering drawings for producing centrifuges (long cylinders spun at high speeds to enrich uranium), but also sold Iran the centrifuges themselves.

But exactly what type of centrifuges did Khan provide? We know he sold the mullahs 100-plus of the older, less efficient P-1 (Pakistani type 1) centrifuges, now in use at the Natanz enrichment facility in central Iran.

The fear is that he also sold them Pakistan's next generation of centrifuges, the P-2. These more advanced tools would significantly truncate Iran's timeline for enriching uranium to "weapons grade" for use in a bomb.

And Khan may have gone beyond help with enriching uranium, by providing highly technical engineering drawings for machining highly-enriched uranium metal into the "pits" required for a nuclear weapon.

Another big question is the nature of Iran's program. U.N. inspectors recently found traces of enriched uranium on equipment from the Lavizan-Shian defense complex near Tehran, which Iran bulldozed in 2004 before IAEA officials could inspect it.

The Iranians, of course, claim the residue and machinery is Pakistani. So why was the enrichment equipment found at a military base? Tehran has long insisted that its armed forces are not involved in its nuclear program.

There are many questions for Khan. He helped North Korea and Libya with their nuclear-weapons programs. While Tripoli has now given up its nuke program, Pyongyang's is still active. And North Korea will soon test a new ballistic missile, capable of reaching the United States.

According to those in the know, Khan henchmen also visited Syria, Saudi Arabia, Egypt, Nigeria and Sudan - and maybe more. With Khan now 71 and rumored to be in poor health, the world may be running out of time to get him to shed some critical light on all these matters.

Because Pakistan's government won't cooperate, U.S. intelligence and the IAEA are still trying to fully unravel Khan's global proliferation network - parts of which are still active, according to experts.

Why is Pakistan resisting a full Khan confession? To shield others, it's widely believed: A number of Pakistani luminaries, mostly in the intelligence and armed services, were likely complicit in, or knowledgeable of, Khan's nuclear network.

But despite this inconvenient truth (and Pakistan's vanity over the sins of its national hero), the regrettable fact is that Khan - and Pakistan - have made the world a much more dangerous place, from Iran to North Korea.

With all that's at stake over Iran's nuclear program - crisis diplomacy, potential interruptions of Persian Gulf energy flows, economic sanctions and possible military action - more Pakistani cooperation is essential. Failing to come forth on Khan's Persian "peccadillos" only makes the situation more volatile.

Yes, Pakistani President Pervez Musharraf has his hands full with terrorism/extremism right at home, and doesn't need his life made tougher. But "case closed" simply won't cut it.

Pakistan is largely responsible for today's nuclear challenge in Iran. It's incumbent upon Islamabad to make "good" on this "bad" by fully disclosing Khan's - and his associates' - activities with Iran and North Korea - and any others, as well.

Peter Brookes, a senior fellow at The Heritage Foundation, is the author of "A Devil's Triangle: Terrorism, WMD and Rogue States."