Abdul Qadeer Khan has made available documents that he says support his claim that he personally transferred more than $3 million in payments by North Korea to senior officers in the Pakistani military, which he says subsequently approved his sharing of technical know-how and equipment with North Korean scientists.

Khan also has released what he says is a copy of a North Korean official’s 1998 letter to him, written in English, that spells out details of the clandestine deal.

Some Western intelligence officials and other experts have said that they think the letter is authentic and that it offers confirmation of a transaction they have long suspected but could never prove. Pakistani officials, including those named as recipients of the cash, have called the letter a fake. Khan, whom some in his country have hailed as a national hero, is at odds with many Pakistani officials, who have said he acted alone in selling nuclear secrets.