Not having investigated banks I can't speak to the individual crimes, but the big problem is the incentives, which is a result of improper regulation. Investment bankers essentially gamble with people's money. If they win a bet, they're paid massive bonuses. If they lose a bet, they are not usually punished in any way, because it wasn't their money.

The bigger problem historically is that investment banks (which were once a place where very wealthy people could gamble with their money) were allowed to merge with commercial banks (which were a place where normal people store their money to receive a small interest, which was paid for by lending money out at interest in the form of mortgages or purchasing government bonds). When this happened, suddenly investment banks had access to incredibly huge amounts of money to gamble with, and the more they consolidated, the more problematic it would be if they failed.

The "other people's money" attitude is really problematic with big banks, because it means that they can lose investor's money, or even the public's savings, on risky investments, but if they lose enough, then the public will have to pay again (in the form of a tax-funded bailout).

It would be a bit like if I said I'd pay you $1 tomorrow if you gave me $100 today, but instead of using the $100 to make a safe profit, I recklessly gambled $50 of it, lost it, and the next day said, actually instead of paying you $1 can you and all your friends please give me some more money, otherwise I can't even give you your original money back.

The crimes will come from the general attitude, which comes from how things are regulated. Individuals will definitely have falsified how much money they'd lost, and probably taken risks with money they weren't legally entitled to wager, but the bigger problem is institutional and legal.