Who killed Zia?

I have heard that classified documents are normally released by the U.S. government thirty years after the events relating to them, yet more than thirty years have passed since Zia and others were killed in an air crash, but no documents about the event have been released or de-classified. 

If you ask anyone in Pakistan who they think killed the president, most of them unhesitatingly say "CIA". Some of them say it so confidently that I'm tempted to ask them if the CIA consulted them before arranging the crash.

Eric Margolis, the Toronto-based journalist, has repeatedly claimed that it was the Soviets (Russians) who did it. As proof, he pointed to the way Chechneyan terrorists were overpowered in a theater in Moscow. Cans of cold drinks were given to them and among the cans there were a few which contained the nerve gas to incapacitate the guerillas. The cans were opened by remote control, the gas was released, and the terrorists overpowered. This was the way the pilot of Zia's plane was incapacitated. 

I read more on the subject and found that the KGB teaches its students Zia's murder as a case study and how the agency arranged for it to happen. A Russian defector to the U.S. also revealed all the details.

But when I tell this to the few intelligent men I know, they refuse to believe it. The most intriguing thing is what I read today in my grandson's text book ("History and culture of Pakistan" by Nigel Kelly). According to this book, an anonymous caller phoned a daily newspaper two and a half hours before the crash and asked if Zia's plane had crashed. I have never heard about this before, and it seems that our children are being told something that is false.