Former Goldman Sachs trader Fabrice Tourre leaves for a lunch break from Federal Court July 24, 2013 in New York. (STAN HONDA/AFP/GETTY IMAGES)

At 28, Fabrice Tourre was the “deal captain” at Goldman Sachs who helped cobble together and market a complex mortgage product that raked in $1 billion for a prominent hedge fund client.

But other investors lost about that much on the 2007 deal, and the Securities and Exchange Commission, in one of its highest-profile court cases emanating from the financial crisis, alleges that Tourre is partly to blame because he lured those investors into an arrangement that was secretly designed to fail.

On Wednesday, the SEC called Tourre to the stand in a federal trial in Manhattan that is expected to last through early August, and tried to extract from him an admission that he knowingly failed to make key disclosures to some investors.

But Tourre did not go there. At one point, he said he did not intend to mislead anyone, but he conceded that a document he had written to a key player in the deal contained some information that was not accurate.

In civil trials such as this, it is typical for the plaintiff to call a defendant to testify against himself, and highly unusual for the accused to assert a Fifth Amendment right, legal experts said. If Tourre were to refuse to testify, the jury could have been instructed to infer that he would have hurt his case if he had answered the SEC’s questions.

Tourre has attracted attention in part because of some colorful e-mails previously released by the SEC and his former employer, Goldman Sachs, which is paying for his defense. In one e-mail, Tourre calls himself by his nickname, “Fabulous Fab.” Now 34, he is a graduate student in economics at the University of Chicago.

For two hours Wednesday, the SEC grilled Tourre about what he did and did not communicate to investors, including ACA Financial Guaranty, the third-party firm that Goldman Sachs hired to assemble the mortgage product.

The SEC alleges that Tourre and Goldman created the mortgage product at the request of the hedge fund Paulson & Co, which was looking for a way to bet against a drop in the housing market. But Tourre never told ACA of Paulson’s strategy, leading ACA to believe that its incentives were aligned with Paulson’s when they weren’t, the SEC said.

The SEC also alleges that Paulson played an influential role in picking the securities that went into the mortgage product, which Tourre failed to disclose to investors.

Without admitting or denying wrongdoing, Goldman settled the case for a record $550 million in 2010, but Tourre refused to do so.

On Wednesday, Tourre conceded that a document he wrote about the structure of the deal contained problematic language. That document was forwarded in an e-mail to Laura Schwartz, an ACA executive.

“It was not accurate,” Tourre said. “I had no intention to confuse anyone.”

Earlier, Schwartz had testified that her company would have never agreed to put together the deal, or invest in it, had it known that Paulson was rooting for the product to fail, which it did when the housing market tanked.

“I am sure that I was never told,” Schwartz said.

But when questioned by Tourre’s attorney, John “Sean” Coffey, Schwartz also said she could not recall any specifics about her conversations with Tourre about the deal when they first met in January 2007.

Tourre, a Frenchman and engineer by training, took the stand soon thereafter wearing a black suit and purple tie. He leaned into the microphone when he spoke as directed by U.S. District Judge Katherine Forrest, but often had to repeat words for the court reporter.

“Sorry, my French accent,” he said when asked to repeat the word “bond,” eliciting some chuckles from the audience.

Tourre is scheduled to continue his testimony Thursday, and legal experts said his performance could make or break his case. “The most important issue for him on the stand will be believability,” said Jacob S. Frenkel, a former SEC enforcement lawyer and a former federal prosecutor. “If he does well, that can equate with a win. If he does poorly, he can collapse his own defense.”

Stuart Slotnick, a defense attorney in New York, said Tourre would benefit if he can make it through the next few days without sparking fireworks.

“The jury expects the SEC lawyers to bring out the smoking guns,” Slotnick said. “In order for the SEC to prevail, they’re going to need to hit a home run.”

Researcher Eddy Palanzo contributed to this report.