Now it can be told! Twenty years after the great stock market crash of October 19, 1987, when the Dow Jones Industrial Average fell by more than 22% in a single day, the truth about why it happened can now be revealed.

And believe me, I know what really happened. Because I caused the crash myself. Yep, it was me. With a little help from my some bad judgment, some colossal mistakes, and some outright criminality.

Back then I ran portfolio management and trading for Wells Fargo Investment Advisors. With $69 billion under management, we were then the world's largest institutional investment manager. The company is now called Barclays Global Investors, and with almost $2 trillion under management, it's still the world's largest.

In 1987 Wells Fargo was by far the largest player in the two strategies that caused the crash. One was "program trading," the simultaneous execution of hundreds of stock trades with a single electronic order. The other was "portfolio insurance," a hedging strategy that used program trading and stock index futures to hedge the downside risk in institutional stock portfolios.

According to official government reports issued in the aftermath of the crash, program trading and portfolio insurance combined to cause the crash. The portfolio insurance strategy required that Wells Fargo execute program trades, selling all 500 stocks in the S&P 500, over and over as the market declined. With every execution, the market declined even more. And that triggered the next execution.

Wells managed portfolio insurance strategies for America's largest pension plans. But other giant pension plans just used us to manage their S&P 500 index funds, managing the portfolio insurance part themselves. On the day of the crash, we were executing plenty of sell programs for our own portfolio insurance strategies all day. At the same time, clients called to say, "Sell a billion S&P, right now" -- and we did. Then they called again. And we sold again. And again.

That's why I say I caused the crash. It was my team's finger that was on the sell button. It was our job to push it, and we pushed it.

Our execution of portfolio insurance and program trading strategies were blamed for the crash in the official government reports. But here's what got left out of the official story: the whole thing was triggered by a massive market manipulation by a major Wall Street investment bank. Lest I end up in litigation about this, that firm will remain nameless.

Here's how it happened.

Before the crash, it was widely discussed among professional investors that the combination of portfolio insurance and program trading could cause a cascading market decline, as each step downward caused portfolio insurance strategies to do more program selling, which in turn would cause a steeper decline, and more selling, and so on.

One especially aggressive head trader on the proprietary trading desk of one particularly aggressive investment bank had followed that discussion, and it gave him an evil idea. What would happen, he wondered, if he started massively short-selling stock index futures, driving the stock market down single-handedly as long as he dared to sell enough contracts -- thus setting off the cascade. Once the cascade was set in motion, he could keep selling, knowing that the portfolio insurers would drive the market lower and lower. Eventually he'd cover his shorts at a huge profit.

It worked. And this man was in a great position to know that it would. His firm acted as broker for all the largest portfolio insurers, including Wells Fargo. So he knew exactly how much the market would have to decline to set off the portfolio insurers' sell programs. In other words, he had inside information. All he had to do to make a fortune was use that information, betray his own clients' trust in his firm, and threaten the world financial system by causing the biggest stock market crash in history.

So the crash of Monday, October 19, 1987, was started as a crime. But it ended as a comedy of errors the next morning, Tuesday, October 20.

Monday night after the market closed, hedge fund mega-manager George Soros became convinced that the world stood on the brink of a global depression. He felt sure that the downward spiral begun on Monday would continue, and he resolved to get out of stocks as quickly as possible.

An hour or so after the market opened on Tuesday morning, he placed an order to sell about $1.6 billion in stock index futures. It was a market order, which means he didn't specify a price -- just sell, no matter what.

By today's standards, $1.6 billion may not sound like much. But in 1987, that was about as big as a trade could get. But then, something astonishing happened that made the trade even bigger.

Somehow, in all the pandemonium on the trading floor in Chicago where the stock index futures were traded, Soros's order got doubled. The ticket got printed twice. So instead of a sell order for $1.6 billion, it was $3.2 billion. All at once. At the market.

His order hit the futures market like a boulder dropped in a shallow pond. Over the next hour, the futures fell almost 27%, an even larger drop than the market had sustained in the previous day's historic crash. The drop in the futures market caused renewed panic in the stock market, and for much of the day it looked like Soros was going to be right -- the meltdown appeared to be continuing.

But he was right for the wrong reason. It was a mistake. A mistake of judgment to sell so much in the first place (Soros now remembers that as one of the worst trades he ever made¬) and another mistake on top of that for his broker to sell twice as many contracts than Soros had ordered. By the end of the day, the stock market and the futures market found their footing, and the foundation for a sustainable recovery was in place.

The crash, then, was an artificial creation -- the product of error and manipulation. It wasn't a actual investor expectations about earnings, the economy, or anything else. So it caused no lasting harm to investors -- stocks actually showed a positive return in calendar 1987, all told. And it did no lasting damage to the economy.

Maybe some day the true story will be told about the panic that markets have been through the last three months. This time around I didn't have a front-row seat like I did in 1987, so I'm not in a position to know that story. But I'll bet there's one out there, waiting to be told.

Like the 1987 crash, I'll bet the sub-prime meltdown will enter the history book as little more than a curiosity -- no lasting harm to investors, and no lasting damage to the economy.