NEW YORK -- Through the unmarked double doors, past the mountains of cardboard boxes, beyond the rows of empty desks and lifeless telephones, Setrag Mooradian sat alone in his glass-paneled office, watching his computer screen blink.

"It's sad to see it like this," he said.

One year ago, the desolate Manhattan offices in which Mooradian sat belonged to the most influential stock speculator on Wall Street: Ivan F. Boesky. The place was alive then with the exuberance of profit-making -- two dozen employes manned the telephones and managed the voluminous paper work generated by Boesky's huge stock trades.

These days, Mooradian is about the only one left. And the most important work he does now is not for Boesky, but for a phalanx of federal investigators who are in the midst of the most ambitious probe of securities law violations in U.S. history. (Citing the advice of his attorney, Mooradian declined to discuss any aspect of his cooperation with the government's investigation.)

An accountant who worked for Boesky for more than a decade, Mooradian is one of several people -- along with former Boesky stock trader David Jachimczyk, Drexel Burnham Lambert Inc. senior vice president Charles A. Thurnher, and others -- said by sources to have become key witnesses in the ongoing investigation sparked by Boesky's guilty plea to securities law violations and his unprecedented cooperation with the government.

That cooperation was disclosed to a stunned public one year ago this week. On Friday, Nov. 14, 1986, in a press conference after the market had closed, the Securities and Exchange Commission announced that Boesky had agreed to pay the government $100 million to settle civil charges that he traded stocks illegally on the basis of confidential information about upcoming takeovers.

The government also disclosed that Boesky, after negotiations with the SEC and Manhattan U.S. Attorney Rudolph Giuliani, had agreed to plead guilty to one criminal count. Suddenly, the man viewed by many as the most important stock trader of his era faced up to five years in prison.

Anything seemed possible.

The following week, frightened traders dumped stocks of companies involved in takeovers amid rumors that Boesky had been secretly tape-recording conversations with his Wall Street colleagues. Wall Street buzzed with rumors that Boesky's cooperation could lead to charges against Drexel and its influential financier, Michael Milken. Milken's central role in the corporate takeover economy -- he has raised billions of dollars for takeovers through the sale of risky, high-yielding junk bonds -- added to the nervousness.

One year later, while the wide-ranging government investigation of Milken and Drexel continues, no charges have been filed against them, and uncertainty about the case persists. Drexel repeatedly has said it knows of no wrongdoing by the firm or its employes, including Milken.

At the heart of the Drexel investigation are statements made by Boesky to the government about an alleged improper stock trading arrangement with Milken that Boesky has said was designed to facilitate corporate takeovers. In addition, various other Drexel transactions, which have no apparent link to Boesky, are being scrutinized.

As described by Boesky and other cooperating witnesses, the alleged arrangement involved "stock parking" in connection with a large number of takeovers, sources say. Under the alleged arrangement Boesky described, he would buy and hold stocks he was instructed to purchase, but he was not always the true owner since he allegedly was protected against some losses by Milken.

Boesky has told investigators that his purchases were designed to help Milken complete takeover deals for Drexel clients and that the arrangement, which involved sharing of profits and losses, was reconciled by a $5.3 million payment from Boesky to Drexel in March 1986.

Drexel has said that the $5.3 million payment was a legitimate investment banking fee that the firm earned doing work for Boesky. Under siege for a year from several sides, top officials at the firm said they still have no idea when the probe will be completed.

"We're waiting to hear," Drexel Chairman Robert Linton said last week. "We've supplied a ton of documents and a lot of testimony and it is up to them. I don't know if the passage of time is bullish or bearish. We have never been involved in something like this before."

Although the government's investigation of Drexel has been widespread, prompting some observers to question whether the probe has become unfocused, events during the last year provide strong indications about where the case is headed.

During that year, prosecutors have twice made public disclosures about the course of their investigation: when Boesky entered his guilty plea last April, and when the SEC settled charges with Mooradian, Boesky's accountant, in September. Both times, the government has revealed details about Boesky's alleged illegal trading arrangement with Milken.

Boesky pleaded guilty to conspiring to file false statements with the SEC in connection with the takeover of Fischbach Corp., a New York-based electrical contractor. Documents released by prosecutors referred to "a secret agreement among the {unidentified} conspirators that {Boesky} would be made whole for any losses" he incurred while purchasing Fischbach stock in advance of the takeover. Sources said that one of the conspirators referred to in the documents was Milken. Boesky's purchase of Fischbach shares, sources said, assisted another Milken client, Victor Posner, in his takeover bid for Fischbach.

Moreover, when the government announced the settlement of civil charges against Mooradian, it again disclosed details about a conspiracy involving Fischbach. SEC papers said that a Boesky entity "acquired and held certain securities of Fischbach Corp. and other issuers in which one or more unaffiliated persons had beneficial interests" in violation of securities laws. Sources said that one of the "unaffiliated persons" was Milken.

These disclosures have been paralleled by behind-the-scenes prosecutorial maneuvers that also provide a picture of the government's case. At least three witnesses in the investigation are said by sources to have been granted immunity from criminal prosecution as a means of obtaining their testimony. Immunity grants are not made lightly by prosecutors, and they are usually given to witnesses believed to possess important evidence.

The key witnesses known to have been granted immunity so far, according to sources, are Mooradian, Drexel accounting executive Thurnher, and former Boesky trader Jachimczyk. Each of them, sources said, possesses some details of the trading arrangement Boesky has described to the government -- information that could be critical if the government decides to bring charges based on the "arrangement."

Prosecutors probably would be reluctant to charge Milken if the case turned solely on Boesky's testimony, sources say. Boesky's plea bargain and admitted crimes would make him vulnerable to attacks on his credibility by defense lawyers.

One factor in any assessment of the case's strength -- a factor about which little is known publicly -- is the content of the secret tape recordings that sources said Boesky made after he began cooperating with prosecutors but before his plea bargain was disclosed. Some sources said that whatever tapes Boesky made relating to the alleged trading arrangement, if any, do not appear to have provided the government with irrefutable evidence.

The search by prosecutors for corroborating evidence also has focused increased attention on immunized witnesses such as Mooradian, Thurnher and Jachimczyk.

Sources described Jachimczyk as the most important cooperating witness to emerge so far. A quiet assistant stock trader who worked in Boesky's midtown Manhattan offices and who once appeared in a rock music video, Jachimczyk is said by sources to have executed many of the stock trades involved in Boesky's alleged arrangement with Milken and to have detailed, contemporaneous records of the trades -- records that contain repeated references to Drexel.

Sources said Jachimczyk has told investigators he was aware of the alleged arrangement while it was occurring. In addition, sources said he has turned over his records relating the alleged arrangement to the government. Jachimczyk reported to former Boesky trader Michael Davidoff, who pleaded guilty earlier this year to securities law violations and who also is believed to have corroborated aspects of Boesky's testimony about the alleged arrangement with Milken. Sources said Boesky was not aware that Jachimczyk had maintained the records.

Investigators are also said to be keenly interested in the testimony of Mooradian and Thurnher, who according to sources kept accounting records of the arrangement for Boesky and Milken, respectively.

Mooradian and Thurnher discussed accounting aspects of the arrangement about a dozen times by telephone, according to sources, and on one occasion Mooradian traveled to Beverly Hills to meet with Thurnher and reconcile their accounts.

Sources said Mooradian has told investigators that he was aware the stock trading arrangement was illegal, but that neither he nor Thurnher knew about the arrangement's origins or purpose. Sources said Mooradian's knowledge of the alleged arrangement appears to be limited to its financial aspects.

But the sources also said that the precise financial details of the arrangement were not always clear. The sources said that in some cases, Boesky held one block of stock in a takeover target company that was subject to the terms of his alleged secret deal with Milken, and another amount of stock in the same company that was not part of the arrangement. The result was sometimes confusion for Thurnher and Mooradian over how to account for the financial aspects of the trading, sources said -- confusion that could weaken their impact as witnesses if there are future cases that go to trial.

Disclosure of the roles allegedly played by Mooradian and Thurnher in the trading scheme, and their position near the center of the continuing federal investigations, have heaped a variety of pressures on two men who have spent their careers far from the public spotlight.

Mooradian is described by former colleagues as a veteran, well-liked, and loyal employe who carried out Boesky's instructions without question. In a brief interview on topics unrelated to his role in the government's investigations, he chatted amiably about the stock market while chain-smoking cigarettes and sipping coffee from a paper cup.

In age, demeanor and life style, Mooradian veers far from the modern Wall Street image of excitable, trend-conscious youth. Resident of a modest, settled New Jersey suburb, he is said by friends to be a connoisseur mainly of jelly doughnuts and smoking. As part of his settlement with the SEC, Mooradian has been banned for one year from employment in the securities business.

Although this is his most serious infraction, this was not Mooradian's first run-in with the SEC; in 1963, Mooradian was charged with violating various federal securities laws in connection with the failure of Fliederbaum, Mooradian & Co., a brokerage firm he founded the previous year. In 1977, the New York Stock Exchange fined Mooradian $20,000 and suspended him from employment in the securities business for three months. NYSE documents said Mooradian was fined because his answers to questions about his past were "either false or incomplete."

But far from criticizing Mooradian, government officials praised him when his settlement was announced in September. "His cooperation has been helpful," SEC enforcement chief Gary Lynch said at the time. "Under his settlement with us, that cooperation extends to the future, including testifying in depositions or at trial, if there is ensuing litigation.

Like Mooradian, Drexel's Thurnher has found himself the object of unsought public attention during the last year. And the pressures on Thurnher are complicated by his continued employment in Drexel's Beverly Hills office, even while he deals with the government as a witness who has immunity from criminal prosecution.

Thurnher's title with Drexel is manager of administration and operations and corporate senior vice president. His duties have included overseeing many of the administrative functions -- including accounting -- at the Beverly Hills office.

The boyish-looking Thurnher, known to his colleagues as "Charlie," lives in an affluent section of the San Fernando Valley in a home purchased for about $585,000 last year; ironically, the purchase was recorded on May 12, 1986, the day former Drexel investment banker Dennis B. Levine was arrested. (Levine's cooperation led government investigators to Boesky.)

Thurnher, who has a reputation as a devoted family man and a loyal employe, resides in a private neighborhood with a security guard manning the entrance. Sculpted trees line the driveway leading up to his home; weeping willow trees and stables are nearby.

Sources said that over the years, Milken has placed considerable trust in Thurnher, assigning him responsibility for keeping records of trading, compensation, personnel and other matters in the Beverly Hills office.

High-ranking Drexel officials said they do not know what Thurnher has told government investigators about the records he maintained for Milken and Drexel. "I have no idea what he said or what they asked him or anybody else they questioned," Drexel Chairman Linton said.

The Drexel case is not the only product of Boesky's extensive cooperation. During the past year, he also fingered former merger specialist Martin A. Siegel and former brokerage chief Boyd L. Jefferies, two leading figures of the 1980s' era of takeover-driven finance.

Both men await sentencing and their cooperation has triggered other investigations. One of those ongoing probes -- of three prominent Wall Street executives accused of insider trading by Siegel -- has sparked controversy because the government dropped its charges after arresting the men. Prosecutors have vowed to refile the case, but no new indictment has been handed up.

Despite these highly publicized cases, it is Boesky's involvement in the Drexel-Milken investigation that is widely regarded as the most important aspect of his cooperation, not only because allegations in the case go to the center of the controversy over takeovers, but also because the Drexel case is viewed as a primary justification for Boesky's extraordinary plea bargain with prosecutors.

One year after the announcement of his deal, Boesky remains a free man. He once courted the press and published a book about how to profit by speculating in takeover stocks, but in the last year he has tried to maintain a low public profile. In spite of his efforts, a steady stream of publicity has surrounded him.

Following the announcement of his settlement with the government last November, Boesky continued to work at his Fifth Avenue office in Manhattan each day, assisting in the dismantling of his financial empire. More recently, sources said Boesky has been doing volunteer work through a New York charitable organization under an assumed name -- after unsuccessfully attempting to secure a volunteer position with organizations affiliated with the United Jewish Appeal (UJA). Boesky had supported UJA financially for several years.

But much of Boesky's time these days is spent as a student at the Jewish Theological Seminary, according to Rabbi Wolfe Kelman, Boesky's guidance counselor at the Manhattan school. Kelman said Boesky is taking several courses, but is focusing his efforts on mastering the Hebrew language. "He is applying all that energy and all that talent to the study of Judaism, which I find quite remarkable," Kelman said. "I know he does his homework. He is responding in a very positive way, with a great deal of joy, to his studies. He is not sullen. He gets tremendous satisfaction out of passing a test or doing well.

"I have been impressed by his emotional strength. I have seen other people in comparable situations who have not had this strength of character. He just doesn't sit about and moan and weep in self-pity. ... He is treated like any other student, although everyone knows who he is and what awaits him."

Boesky has a sentencing date next month, but sources said it is likely his sentencing will be postponed until the full benefits of his cooperation in the Drexel matter become clearer. Sources said it could be 1988 before a final decision is made by the Manhattan U.S. attorney and the SEC on any charges concerning Drexel and Milken.

While Boesky waits, so does Wall Street. Speculation about his cooperation and the progress of the Drexel case have become a favorite topic of conversation among interested observers.

"One interpretation is that they {government investigators} are having difficulties lining up the kind of solid case they would like to have," said former SEC commissioner A.A. Sommer Jr. Or, he continued, "it could mean conceivably they are developing a monster case, something very widespread with all kinds of ramifications. ... If I had to say which is more likely, that is more likely."

The employes in Drexel's Beverly Hills office have remained intensely loyal to Milken in the year since the investigation of the firm began. Many are young men who have attained wealth overnight through the sales and trading of high-yielding junk bonds, which have been used to finance growing companies and corporate takeovers.

Sources said Milken's operation has continued to prosper in the past year. But under the cloud of the continuing government investigation, its future is said by many to be less certain.