They are scribblings that may come back to haunt Matthew Tannin. The former high-flying Bear Stearns hedge fund manager – who goes on trial for fraud in a New York court this week – had a habit of recording his inner-most thoughts in emails sent to himself on a private Google Mail account.

"I am going to use this to keep my diary," he wrote. "I didn't want to use my work email any more."

In words never intended for public consumption, Tannin wrote of his worries about becoming dependant on an antidepressant, Wellbutrin, and a stress medication, Lorazapan, to cope with concern about the performance of his fund. He expressed satisfaction at earning close to $2m (£1.3m) in a year but alluded to a "religious crisis" and complained about "schlepping the kids around from place to place" during a holiday in London.

As his confidence in his money-making panache began to falter, Tannin pinpointed a meeting in 2006 when he realised that his Bear Stearns fund faced potential trouble: "I had a wave of fear set over me – that the Fund couldn't be run in the way that I was 'hoping'. And that it was going to subject investors to 'blow up risk'."

Tannin and his boss, Ralph Cioffi, ran two funds holding $1.4bn of clients' funds that collapsed in July 2007, an event widely viewed as the first clear signal of America's sub-prime mortgage crisis and the global credit crunch. The meltdown of these funds sparked a chain of events that contributed to the demise of Bear Stearns, an 85-year-old Wall Street institution, in early 2008. They have been charged by US prosecutors with defrauding customers by hiding the true condition of investments as prospects steadily darkened.

The first high-rolling financiers to face criminal action arising from the financial crisis, Cioffi and Tannin have become unwitting poster boys for perceived arrogance, recklessness and irresponsibility on Wall Street. Frustrated at not seeing higher-ranking bank bosses clapped in irons, the public and the US media are watching keenly.

"I do think there's a desire on the part of the public to see people held accountable," said Barbara Roper, director of investment protection at the Consumer Federation of America. "The trouble is that a lot of what brought down the system was legal."

According to the government, Tannin and Cioffi stuffed their funds with dangerous mortgage-linked securities while marketing them as low-risk, high-quality investments. Federal authorities obtained Tannin's deleted email account by serving a subpoena on Google, forcing the company to search its archives.

Prosecutors say the pair realised at an early stage that things were going amiss, exchanging messages remarking that conditions had turned "pretty damn ugly" and that the sub-prime market was "toast". But they constantly reassured customers that they were comfortable, that there were buying opportunities and that there was no cause for alarm. Behind the scenes, the US government contends that Cioffi's private concern was such that he withdrew $2m of his own money, reducing his own "skin in the game".

The trial, which begins on Tuesday, is expected to last six weeks, with at least 38 prosecution witnesses and 500 exhibits. Arrested in June last year, Cioffi and Tannin have had 16 months to prepare their defence. Their lifestyles have been widely scrutinised – New York magazine recently reported, in an unsympathetic tone, that Cioffi had been obliged to sell his beachside retreat in the Hamptons and two of his three Ferraris.

Legal experts say that it will be a tough case for the government to prove. Few of the facts of what happened are in dispute. But prosecutors must convince a jury of the defendants' state of mind by producing evidence of intent to defraud.

"The government will need every drop of evidence it has to prove intent," said Peter Henning, a white-collar law expert at Wayne State University in Michigan. "These cases are circumstantial. It's a string of inferences. It's about what they knew and when they knew it."

They may be the first. But Cioffi and Tannin will by no means be the only financiers to face criminal proceedings arising from the credit crunch. The FBI has more than 580 corporate fraud investigations underway, of which at least 40 concern sub-prime mortgage lending.

"New York white-collar lawyers are doing quite well right now responding to grand jury investigations and the threat of grand jury investigations," said Daniel Richman, a professor at Columbia Law School. "There's a sense that quite a few more are moving down the pipe."