It’s been five years since the worst days of the financial crisis, and the government has been unable to hold any prominent Wall Street executives responsible for the enduring damage to the global economy. The most common explanation for this failure: some on Wall Street may have behaved unethically, but they didn’t violate any laws. Watching the Securities and Exchange Commission in action in the trial of a former Goldman Sachs trader, a more basic explanation presents itself: the government just hasn’t been very good at bringing fraud cases against those who may have committed crimes during the financial crisis.

The Securities and Exchange Commission points out that it took fifteen cases to trial in 2012 and won thirteen of them, including an $8.9 million judgment in a case against Big Apple Consulting USA and its executives. But when it comes to holding Wall Street executives accountable for the financial crisis, the S.E.C.’s record has been unimpressive. The Commission has failed to bring any cases to trial against Wall Street firms, opting instead for settlements in which companies neither admit nor deny their guilt. The cases brought against individuals have involved lower-level executives and haven’t been successful. Last summer, the S.E.C. lost a case against Brian Stoker, a former Citigroup manager. In November, it dropped charges against Edward Steffelin, a former GSC Capital Corporation executive, over a defaulted collateralized debt obligation issued by JPMorgan Chase.

Now we’ve come to the deciding moment in the latest high-profile case.

On Tuesday, the S.E.C.’s chief litigation counsel, Matthew Martens, gave his closing arguments in the government’s civil-fraud case against Fabrice Tourre, a Goldman Sachs trader accused of cheating investors out of more than a billion dollars six years ago in a mortgage-backed-securities deal known as Abacus. Goldman has already paid a $550 million fine to the S.E.C. to settle legal claims related to the Abacus affair.

Tourre is a relatively junior trader, an elegantly dressed Frenchman with a colorful personality; at the time of the deal, in 2007, he was just twenty-eight, and the allegation that he had played a crucial, illicit role in such a large and disastrous set of transactions made him a visible sign of everything that was going wrong on Wall Street. The case of Tourre, nicknamed “Fabulous Fab,” also represents one of the government’s last chances to snare a culprit from the financial-crisis era.

Martens’s summation on Tuesday was forceful and eloquent. It also repeated tactical errors the S.E.C. made throughout the trial.

For example, he called attention to Tourre’s embarrassing late-night e-mails to his then-girlfriend Marine Serres in an attempt to show that Tourre intended to commit fraud. In one e-mail, Tourre mixed romantic talk with a wonky analysis of the unsettled state of the mortgage markets in early 2007. In another, which could only be classified as French humor, Tourre rewrote the lyrics of Charles Aznavour’s 1965 hit “La Bohème” in a parody that involved mortgage traders gathering around a stove. In other e-mails, Tourre joked about “monstrosities” he had created and about selling worthless securities to widows and orphans. But the tabloids made better use of Tourre’s excessive and sometimes tasteless flourishes than did the S.E.C., which overplayed its hand. The e-mails, for which Tourre expressed regret and personal embarrassment in testimony, seemed more likely to humanize him in the eyes of the jurors as a romantic who wrote notes to his “darling” and “sweetheart.” They contributed to a portrait of Tourre that doesn’t fit into a simple morality tale about greed on Wall Street. The timing didn’t help, either, given the concerns about the government making broad use of private correspondence. (Approached after making his closing argument, Martens declined to comment on criticism of his trial strategy. A spokeswoman for the S.E.C. declined to comment on how it has handled fraud cases in general.)

Here’s the question at the heart of the case: Is it fraudulent for an investment bank such as Goldman Sachs to fail to disclose, in sales materials, information that a prudent investor would consider important to an investment decision, even if the client reading the sales material is sophisticated?

To understand why the government presented its case so ineptly, one needs to understand the Abacus deal. Abacus created what is known as a “synthetic” portfolio of residential-mortgage-backed securities. Customers could bet that the securities would go up or down. Goldman marketed a “long” position in Abacus to clients like IKB Deutsche Industriebank, a German bank that thought the value of the portfolio would go up. The deal closed in April of 2007, at a time when only Goldman and a few hedge funds were “shorting” the mortgage market—that is, betting that it would go down. Those short bets paid off handsomely for Goldman, which in 2007 reported net earnings of $11.6 billion just as practically every other bank on Wall Street was edging toward insolvency because of their “long” bets on the mortgage market.

The case against Tourre stems from his role in preparing the documentation and marketing materials for Abacus, which stated that the portfolio was selected by ACA Management LLC, an affiliate of ACA Financial Guaranty Corporation. In fact, the portfolio’s history was more complicated. The previous summer, hedge-fund manager John Paulson had raised hundreds of millions of dollars with the intention of betting that the value of subprime residential-mortgage-backed securities would decline as people started defaulting on their mortgages—that is, that the value of securities like those he had designated would fall. Paulson asked Goldman to create a transaction that would let it take a short position in the subprime residential-mortgage-backed securities market. Goldman and Paulson then selected ACA as the portfolio-selection agent for what would become the Abacus 2007-AC1 transaction. In May, 2007, ACA invested its own capital in Abacus, betting that the portfolio would go up in value. ACA was the largest “long” investor in Abacus, and it lost $951 million when the portfolio suffered catastrophic losses later that year. (ACA has filed a lawsuit against Goldman, which has been dismissed by a New York appellate court but which ACA has vowed to appeal to New York’s highest court.)

To make its case, the S.E.C. presented some preliminary e-mails from early January of 2007. But it wasted four days on testimony from ACA employees, who admitted they couldn’t remember much about how ACA came to its mistaken impression about Paulson’s interests. In the meantime, there were investors and potential investors who were presented with marketing materials that failed to disclose Paulson’s role in selecting the portfolio—and, unlike ACA, had no direct way to learn of his involvement. In his closing remarks, however, the S.E.C.’s Martens spent precious little time making this more compelling fraud case to the jury and continued to pound away at the more tenuous claim that Tourre defrauded ACA in particular.

The S.E.C. also did poorly in calling as witnesses Tourre’s former colleagues and friends in the trading operations of Goldman Sachs. They, like Tourre, seemed to have no idea what sales materials were supposed to contain. Had the S.E.C. called an expert witness or even a witness from Goldman Sachs’s own investment-banking division, the jury might have had enough information to decide whether the Abacus sales materials met the normal legal and ethical standards of disclosure. In fact, in 2011, Goldman Sachs’s own internal investigation of its failings in the financial crisis recommended that, to strengthen its “reputational excellence,” the Securities Division in which Tourre’s unit resided needed to develop sales “policies and standards, approval processes, disclosure requirements and oversight” that matched policies elsewhere in the firm. In making this recommendation, Goldman was admitting the Abacus sales materials did not even live up to its own standards. In fact, the firm moved all mortgage-related-securities sales-material preparation from the Securities Division to the Investment Banking Division, where managers had far longer and deeper ties with customers than the harried traders who ran the Abacus deal. The S.E.C. missed their chance to bring up this crucial development in its own case.