Dr Khan was feted in Pakistan as a hero for his nuclear work
AQ Khan is allowed few visitors these days.
The large house in a plush district of Pakistan's capital, Islamabad, that was once his home is now his prison.
Dr Khan has been confined to house arrest since his confession in February 2004 that, as the man who had helped deliver the nuclear bomb to his native Pakistan, he had gone on to transfer nuclear secrets and technology to an array of countries around the world.
Next to the main building sits a guesthouse in which Dr Khan used to entertain his friends and contacts, including many of the Western businessmen who worked closely with him.
Now, the guesthouse is the site of the security detail that monitors Dr Khan's movements and ensures there is no unauthorised contact between him and the outside world.
One of the few people who had been allowed to visit regularly was his daughter Ayesha.
But for the past five weeks, she has been unable to enter her father's house.
No other family members are allowed to visit.
The BBC has also learnt that other measures have been put in place to tighten security.
Dr Khan shared nuclear technology with nations like Libya and Iran
These include a gate that has been covered up so that no-one can look into the garden and which ensures that Dr Khan cannot look out.
In recent months, other friends and former associates of Dr Khan have been instructed not to talk to journalists or anyone else about him after previously being allowed to do so.
The timing for the tightening of security is mysterious.
It comes at almost exactly the same moment that Pakistani officials announced their investigation of Dr Khan was over.
They also released the last of his former staff who had been held by authorities for more than two years.
Pakistani officials say they have shared all the information they have with international investigators, including those from the UN nuclear watchdog, the International Atomic Energy Agency (IAEA).
The matter is closed, Pakistani officials say, hoping that the world may one day slowly forget about the man the government in Islamabad once feted as a national hero.
Meanwhile, the US has been quietly but consistently pressing for greater access to Dr Khan.
Evidence of nuclear arms transfers to Iran would damage Musharraf
The CIA would like to talk to him direct but Pakistan has resisted, saying instead that questions have to be passed through Pakistani intelligence officers who will then return with the answers.
The official line is that Pakistan should be trusted with the investigation and anything else would be a violation of national sovereignty.
But the suspicion has always been that the authorities are resistant to direct questioning for fear that Dr Khan might suggest Pakistani government or military officials knew of his transfers of nuclear technology to North Korea, Iran or Libya.
The US wants to talk to Dr Khan not primarily to answer the question of Pakistani government involvement but rather to see if the scientist could reveal more about his support for Iran's nuclear programme.
The US particularly wants to know whether Iran is peaceful in intent, as Tehran claims, or geared towards making nuclear weapons, as Washington argues.
In the case of Libya, Dr Khan supplied a nuclear weapons design and investigators wonder whether he could have done the same with Iran.
Confirmation of any such transfer would be highly useful for the US but would also cause embarrassment for President Pervez Musharraf of Pakistan.
But the mystery remains as to why security would be tightened at the house and visits restricted at this time.
Dr Khan himself remains isolated, his health reportedly deteriorating.
Trapped with him are the secrets regarding what exactly he did, why he did it and precisely who helped him.