An SUV sits outside the offices of SAC Capital Advisors in Stamford, Conn. The Justice Department may file criminal charges as soon as Thursday, said to people familiar with the matter. (Douglas Healey/BLOOMBERG NEWS)

The Justice Department will file criminal charges as soon as Thursday against SAC Capital Advisors, once one of the country’s most successful hedge funds, following a multiyear investigation into insider trading, according to people familiar with the matter.

A number of current and former employees of the firm have already been tied to the federal investigation, but prosecutors are getting ready to unveil a criminal case against the legendary firm itself, once exalted on Wall Street for its whopping average annual returns of 25 percent.

Justice officials are also leaving the door open to prosecuting SAC’s famous founder, Steven A. Cohen, as well as other individuals at the firm, according to the people familiar with the case, who spoke on the condition of anonymity because the investigation is ongoing.

In a massive insider-trading investigation that has ensnared several other firms, SAC is the most ambitious target yet. The hedge fund and its billionaire founder are symbols of Wall Street at its most successful — and most excessive.

SAC, based in Stamford, Conn., charges its clients more than the industry standard because its returns are so high, especially when markets are down. Cohen, whose initials form the name of his company, is known for running a cutthroat office. Traders are expected to bring massive profits regularly to the firm, or they are asked to leave. Cohen has been known to do anything to gain an edge; he has hired in-house psychiatrists to hone his employees’ ability to trade while under stress.

He is also known for his lavish spending habits. Earlier this year, he paid $155 million for the Picasso painting “Le Rêve,” the most on record for a U.S. collector. In the same month, he also bought an oceanfront property in East Hampton, N.Y., for $60 million.

Criminal charges against the firm could mark the end of SAC, which has already seen investors steadily pull out their money after its legal troubles began to bubble up a few years ago.

Jonathan Gasthalter, a spokesman for SAC, declined to comment.

The government’s insider-trading investigation, led by Manhattan U.S. Attorney Preet Bharara, is by far its biggest since the late 1980s when prosecutors went after junk bond trader Michael Milken.

While the government has pursued far-reaching investigations of insider trading at hedge funds, there have yet to be any major criminal prosecutions of financial firms at the heart of the 2008 crisis, many of which received massive taxpayer bailouts.

Nonetheless, the insider-trading investigation has already netted a number of high-profile people.

In 2011, Raj Rajaratnam, co-founder of another hedge fund called Galleon Group, was convicted and sentenced to 11 years in prison for his role at the center of an extensive network of consultants, analysts and traders passing nonpublic information between them — and then profiting from it.

That investigation, marked by its reliance on wiretaps, has slowly reached the doorstep of SAC. Former SAC employees Mathew Martoma and Jon Horvath were charged last year with insider trading.

In March, the company agreed to a $616 million settlement with the Securities and Exchange Commission, the biggest ever for insider trading. Also that month, another fund manager at SAC, Michael Steinberg, was indicted by a federal grand jury on counts of conspiracy and securities fraud.

The big question left is whether Cohen will face criminal charges.

Earlier this month, the SEC accused Cohen of failing to supervise Martoma and Steinberg, two of the SAC portfolio managers accused of insider trading. But the agency did not charge him with fraud or insider trading.

In that civil case, the SEC is seeking to demonstrate that Cohen received suspicious information that should have tipped him off to wrongdoing at his firm.