Spurred by the surprise emergence of new evidence on a computer hard drive, the Securities and Exchange Commission has reopened a major insider trading investigation it was strongly criticized for dropping, people with knowledge of the case said.

The inquiry has to do with giant hedge fund Pequot Capital Management and its chairman and C.E.O., Arthur Samberg. Portfolio.com has learned that within the last two weeks the S.E.C. issued a subpoena in the case, a step taken only in a formal investigation approved by senior agency officials.

Reopening the investigation marks a new embarrassment for the beleaguered S.E.C., suggesting that, as in the Bernard Madoff case, it may have failed earlier to follow up adequately on strong indications of possible wrongdoing.

People close to the case said the subpoena is for the hard drive from a computer owned by David Zilkha, a former Microsoft employee who briefly worked for Pequot in 2001. The S.E.C. and other federal investigators already have printouts of e-mail messages on the hard drive.

Copies of the emails obtained by Portfolio.com appear to show Zilkha soliciting nonpublic information about Microsoft from a neighbor who was a more senior official at the software company.

The original S.E.C. investigation, which ended in 2006 without the agency taking any action, had looked into whether Samberg had made highly profitable trades based on confidential information from Zilkha about Microsoft earnings.

The earlier investigation, which had focused on Pequot trading in 2001, drew wide attention after the S.E.C. in 2005 fired the lawyer handling it. The S.E.C. lawyer, Gary Aguirre, contended he'd been fired for political reasons relating to a separate facet of the case: Whether Morgan Stanley C.E.O. John Mack may have given Samberg inside information relating to a planned acquisition by General Electric.

Aguirre claimed he was fired because higher ups at the S.E.C. didn't want him to depose the politically connected Mack. A subsequent investigation by two Senate committees, and by the S.E.C.'s own inspector general, backed Aguirre and found that the S.E.C. was wrong to have shut down the investigation.

Ever since, two Republican Senators, Arlen Specter of Pennsylvania and Charles Grassley of Iowa, have pressed the S.E.C. to reopen the case.

The existence of the hard drive became known through Zilkha's contested divorce case in Connecticut. People close to the case say that Zilkha's now ex-wife had obtained and kept the hard drive from his home computer before they split up.

The drive contains email exchanges between Zilkha and Mark Spain, a more senior Microsoft official, in 2001, when they were both living in Redmond, Washington.

At the time, Zilkha was still working for Microsoft, although Samberg had offered him a job and was pressing him for information on Microsoft.

Copies of the newly obtained emails show, for example, that on April 7, 2001, Zilkha sent Spain an email with the subject line "Any visibility on the recent quarter?" The message said: "Hey there. Have you heard whether we will miss estimates? Any other info? David."