After a year and a half of investigation, examination of millions of documents, hundreds of witness interviews and four hearings covering more than 30 hours of testimony, the Senate Permanent Subcommittee on Investigations, which I chair, has learned much about the causes of the financial crisis that brought about this recession.

Now it’s time for Congress to put that knowledge to use.

Our four hearings dealt with reckless mortgage lending and how those bad mortgages poisoned the financial system; the failings of federal regulators who observed reckless lending but failed to stop it; credit rating agencies that examined investments built on these shoddy loans and gave them AAA ratings; and investment bankers who put their own profit ahead of their customers and the nation.

The conduct we discovered was deplorable.

But surprisingly, few of the people we questioned in our hearings admitted much responsibility.

The CEO of Washington Mutual Bank, who embarked on a risky mortgage strategy that led to the largest bank failure in history, blamed the federal government for not saving the bank from his recklessness. The head of a regulatory agency who gave the bank a free pass defended his inaction.

Credit raters said the failure of investments they had declared solid wasn’t their fault, that no one could have seen the collapse coming.

And when my colleagues and I asked the leaders of Goldman Sachs how they could sell investments that the firm thought were bad deals, while at the same time betting against those investments, these executives said that was just how they do business.

I was appalled, and a lot of other senators were, too, by how Goldman employees could defend practices that I think most Americans find hard to stomach.

Most Americans have no idea how a company can design and build a product, sell that product to its clients and then go bet that the product will fail.

Most Americans don’t understand how investment banks can buy financial instruments that amount to nothing more than casino bets, and use them to introduce such enormous risk into the financial system that the taxpayers have to bail them out.

Legislation now before the Senate would take several important steps to address these problems.

It would make it far more difficult for lenders who make risky loans to unload that risk onto others.

It would consolidate regulatory agencies so that the government has a more accurate picture of what’s happening in the financial system.

And it would bring some sunlight into risky, hidden financial markets where so much of the damage of the crisis occurred.

It is a strong bill.

One improvement I hope to make is embodied in an amendment that Sen. Jeff Merkley of Oregon and I have introduced to limit what’s known as proprietary trading.

That’s when financial firms invest in markets not on behalf of their own clients, but for the firm’s own profit.

Proprietary trading was one of the factors that weakened financial firms leading up to the crisis. At the start of this decade, such trades accounted for a small percentage of bank profits, but in 2009, proprietary trading accounted for more than three-quarters of revenue earned by large investment firms.

Sen. Merkley and I propose that commercial banks – those whose deposits are insured by the federal government – be barred from risky proprietary trading. Other large financial institutions could still trade for themselves, but would have to keep enough money on hand so that if they lose their bets, the bank covers the losses and not the taxpayers. And we would bar investment banks from constructing complex financial instruments, selling them to customers and then betting on their failure, ending an egregious conflict of interest that our investigation revealed.

One thing is clear: It would be irresponsible of us not to act.

Too many Americans, and too many Michiganians, have lost their jobs, their homes or their businesses in this crisis for us to allow another.

If Wall Street won’t take responsibility for preventing a repeat, Congress must.

Democrat Carl Levin is the senior U.S. senator from Michigan and chairman of the Permanent Subcommittee on Investigations.