Steve Marcus/Reuters

Federal authorities are poised to level a criminal indictment against SAC Capital Advisors, the hedge fund run by the billionaire Steven A. Cohen, capping a nearly decade-long insider trading investigation into one of Wall Street’s most prominent firms.

Prosecutors and the F.B.I. in Manhattan are expected to announce the charges in the coming days, according to people briefed on the matter, who spoke only on the condition of anonymity. The move, a rare aggressive action against a big company, could cripple SAC.

It is unclear whether SAC’s lawyers will try to settle at the last minute, though that is an unlikely option at this point. Mr. Cohen is not expected to be charged criminally, though authorities are still contemplating bringing charges against other employees at SAC.

While the legal deadline for filing some insider trading charges may have already passed, authorities are planning to navigate around that requirement by filing a broader criminal conspiracy case against SAC, these people said. As long as one of the trades cited in the case took place in the last five years – and some did – then the government has the power to sweep in older trades to highlight a continuing scheme.

Representatives for the government and SAC declined to comment.

The indictment would come on the heels of the Securities and Exchange Commission’s filing a civil action last week. It accused Mr. Cohen of failing to supervise employees suspected of insider trading. Those employees, Mathew Martoma and Michael S. Steinberg, had been charged with criminal wrongdoing.

In its order, the S.E.C. cited a 2008 e-mail forwarded to Mr. Cohen in which an SAC analyst explicitly stated that he had a “2nd hand read from someone at” the computer maker Dell, a source who provided financial information about the company before its earnings announcement. Minutes after receiving the e-mail, Mr. Cohen sold his entire position in Dell, the S.E.C. said.

In a 46-page document responding to the S.E.C.’s charges, Mr. Cohen’s lawyers said there was an innocent explanation for his not reacting to the suspicious e-mail: he did not read it.

“Cohen has no memory of having seen it and no witness will testify that they discussed it with him,” the lawyers said in the document, circulated internally at SAC and reviewed by The New York Times and referred to earlier in The Wall Street Journal.

Mr. Cohen, the lawyers argued, received an average of 1,000 e-mails each day in 2008. At the time, he apparently opened only 11 percent of the e-mails, though the lawyers did not disclose how they arrived at that figure.

To locate an incoming message, Mr. Cohen would have to look at the only one of his seven computer screens that displays e-mail, a monitor that happened to be “to the far left” of the others, his lawyers argued. Then he would have to “minimize one or two computer programs” to call up his Microsoft Outlook window, which was “reduced” so that Mr. Cohen could see, at most, only five messages at once.

While the document makes a strong case that Mr. Cohen was not knowingly trading on inside information, it is unclear whether it will rebut the S.E.C.’s claims that he did not prevent employees from doing so. The S.E.C. must show that Mr. Cohen did not “reasonably” supervise them.

Mr. Martoma, 39, and Mr. Steinberg, 40, have each pleaded not guilty to criminal insider trading charges and face separate trials in November.

Mr. Cohen’s civil case will play out before an administrative law judge at the S.E.C. rather than in a federal court. On Tuesday, Chief Judge Brenda P. Murray was assigned to the case, and a hearing was scheduled for Aug. 26.

The SAC document, people briefed on the matter said, was adapted from the lawyers’ response to the S.E.C.’s so-called Wells notice that warned of potential charges. It also outlined the arguments that SAC most likely presented in an effort to persuade the Justice Department not to bring a criminal indictment of the fund.

A criminal charge against SAC would likely serve as a death blow to the firm. SAC has already been hobbled by the government’s investigation, with investors in the fund pulling about $5 billion from the fund since the beginning of the year. But an indictment may pressure more investors to pull their money. It could also force SAC’s trading partners, which include nearly all of the largest Wall Street banks, like Goldman Sachs and Morgan Stanley, to suspend business with the firm.

Criminal charges against companies are extremely rare, and the government is reluctant to bring them given the potential collateral consequences. After the Justice Department indicted Enron’s accounting firm, Arthur Andersen, the firm was forced to close and 28,000 jobs were lost. SAC, which is based in Stamford, Conn., has about 1,000 employees.

Before bringing indictments against companies, federal prosecutors consider a number of factors when deciding to bring a case, including the pervasiveness of wrongdoing and the company’s level of cooperation in the investigation.

The Dell e-mails are expected to play a central role in the criminal case.

Even if he was a vigilant e-mail consumer, the lawyers say, Mr. Cohen could argue that the 2008 dispatch did not identify the source of the information about Dell, suggesting that it could have “lawfully” come from an authorized person at the company. The source, the lawyers note, did in fact turn out to be someone from the investor relations department, who has not been accused of any wrongdoing. The lawyers also note that the information in the e-mail “turned out to be wrong.”

Still, SAC made profits and avoided losses of $1.7 million. And once Dell released its earnings, Mr. Cohen sent an e-mail to Mr. Steinberg that said, “Nice job on Dell.”

Mr. Cohen sold his stake in Dell, the lawyers argue, with “good reason.” Mr. Cohen, they said, took the position based on the recommendation of a portfolio manager at SAC, whom people briefed on the matter identified as Gabe Plotkin. Minutes after Mr. Plotkin started selling, so did Mr. Cohen.