As Secretary-General Annan prepares to leave his post at the United Nations, a mystery is surfacing surrounding his apartment on Roosevelt Island, subsidized by New York taxpayers, which is still in use by the family of his brother, Kobina Annan.

The apartment was where Mr. Annan and his wife lived before 1997, when he became secretary-general. The Roosevelt Island home is part of an estate of low-rent state-regulated housing. For years, the Annans saved considerable sums by occupying an apartment meant to help financially strapped low- to moderate-income New York families.

One question Mr. Annan has never addressed is why he and his wife felt comfortable availing themselves of this generous arrangement. Another is how it is that, since Mr. Annan and his wife left that Roosevelt Island apartment 10 years ago to move into the rent-free residence on Sutton Place supplied to the secretary-general, their former low-rent apartment was handed over to be occupied by the family of Mr. Annan’s brother.

This kind of apartment, part of a state-regulated housing development program called Mitchell-Lama, is subject to strict eligibility requirements, involving family size and financial ceilings on combined family income. There is also a requirement that the leaseholder make continuous use of the apartment as a primary residence.

Kobina Annan is currently Ghana’s ambassador to Morocco, a post he has held for some years. His wife, Ekua Annan, features in the Ghanaian and Moroccan press as active in the Rabat diplomatic community. The pair posed together at a gala reception they hosted at their official Morocco residence in 2004.

No one is saying that any of the Annans have broken the law; the regulations for Mitchell-Lama housing allow a certain amount of flexibility once applicants have obtained a lease. But the issue is pertinent because Kofi Annan, whose wife comes from one of Sweden’s wealthier families, has spent years lecturing Americans on how the well-heeled have obligations to those less fortunate. Those low- to moderate-income New York families for whom such accommodation was built face a four-year waiting list.

The effective New York state subsidies for such an apartment appear to be large. Roosevelt Island sits in the East River, with a view of the United Nations. The residences are part of a quiet enclave with a riverside promenade, just a few minutes from high-rent Midtown. The apartment in question, a three-bedroom unit on the ninth floor, appears from the outside to have a river view looking out on Manhattan. At market rates, according to local real estate agents, a three-bedroom apartment on the island currently rents for about $4,500 a month.

A subsidized apartment, like the one linked to the Annans, rents for less than half that amount, or just under $2,000 a month. While some allowance might be made for wear and tear, the current effective taxpayer subsidy for the Annan apartment could, by a conservative estimate, amount to upward of $10,000 a year, or even as much as twice that, which, over a decade, adds up to a significant sum.

Telephone records show that Kofi Annan first moved into Mitchell-Lama housing around 1978, which suggests that by the time he moved out in 1996, he may have secured state subsidized rental rates for himself for a period as long as 18 years — depending on what he did with the apartment while working in Switzerland for part of the 1980s. Over the decade since he moved out, this reporter suggests that a conservative estimate of the effective New York taxpayers’ subsidy for whoever has held the lease there during that time would add up to well over $100,000.

Exact details are hard to come by. Mitchell-Lama authorities, while willing to disclose rental rates, said the identities of individual leaseholders are confidential. The various Annans involved, including the secretary-general, his brother, and his sister-in-law, have refused to answer all but a few questions related to the apartment. Queries to Kofi Annan’s U.N. office asking for such details as whether and when he gave up the lease and how the apartment passed to his brother’s family elicited no reply beyond an email from the secretary-general’s spokesman saying Mr. Annan “has always lived within his means” and that during Mr. Annan’s many years in New York, “he has always been a law-abiding resident.”

Kobina Annan, reached last week by phone at the Ghanaian embassy in Rabat, Morocco, was briefly willing to talk, saying, “That’s my apartment. I still live in New York, and I go on and off.”

He added: “While I’m not there, my son lives there.” But when then asked how long he had held the lease, he abruptly hung up.

At the time of that conversation, the ambassador’s wife, Ekua Annan, was in Ghana, according to an assistant who put through the call to the ambassador. But shortly after Kobina Annan hung up the phone, Ekua Annan called to say her son, Kobina Annan Jr., lives in the apartment, that she is the leaseholder, and that she is “in the U.S. every one to two months.” Asked when and how she obtained the lease, she, too, hung up the phone.

A call that same day to a number currently listed in a New York telephone directory for Ekua Annan at the Roosevelt Island apartment was answered by a man who refused to answer any questions and quickly hung up the phone.

Mr. Annan, after announcing a new policy of U.N. financial disclosure as part of his reforms over the past year, has refused to disclose to the public his own financial records. When Paul Volcker’s investigation into the U.N. Oil-for-Food scandal took a confidential look at Kofi Annan’s financial affairs last year, investigators declared them clean. But Mr. Volcker provided almost no specifics, releasing none of the underlying documents.

His account did not indicate whether the Roosevelt Island apartment ever came up, or, if it did, whether investigators were attuned to the rules of Mitchell-Lama state-subsidized housing. Mr. Volcker’s committee stopped answering questions from journalists after his final report came out in October 2005, and it now appears to have turned over custody of the files to the United Nations’s own legal department.

Some of the apartment’s history can be pieced together from public telephone listings over the years, as well as documents that have surfaced in relation to congressional Oil-for-Food investigations. New York telephone directories show Kofi A. Annan listed at the Roosevelt Island address, 575 Main Street, as far back as 1978, with the phone number 212-759-5576. For some years in the 1980s, he worked for the United Nations in Geneva, Switzerland. He married his second wife, Nane Annan, in 1984 and returned with her to New York. He then appeared in the New York telephone book at the same Roosevelt Island address, but with a different listed telephone number, 212-355-6938, through 1992. Then Kofi Annan’s name disappeared from the directory — about the time he was promoted from U.N. financial management to peacekeeping, where in 1993 he arrived at what was then the world body’s second-highest rank of undersecretary-general.

Interviews given over the past decade by Kofi Annan and his wife — and documents related to his son, Kojo Annan — suggest that Kofi Annan remained in the same Roosevelt Island apartment until he and his wife moved at the end of 1996 to the luxury townhouse on Sutton Place provided free of charge for the secretary-general to live in by the United Nations.

Kofi Annan’s 1992 Roosevelt Island address and telephone number turn up in a fax sent by his son, Kojo Annan, in late 1996 to his employer at the time, a Swiss firm called Cotecna Inspection SA, which later, while employing Kojo Annan as a consultant, won the U.N. contract to inspect goods imported into Saddam Hussein’s Iraq under the Oil-for-Food program. In this fax, dated December 10, 1996, Kojo Annan told Cotecna that he could be reached over the holidays at a Roosevelt Island address in New York: 575 Main Street, Apt. 909, for which he gave the same telephone number as the one listed publicly a few years earlier for his father, Kofi A. Annan: 212-355-6938.

The following month, in January 1997, Kofi Annan became secretary-general and moved out. Later that same year, two new telephone listings appeared for the Annans at the same Roosevelt Island address: Ekua Annan at 212-317-2595 — a number that a Washington Hotel bill shows Kojo Annan calling late in the evening the following year, in October 1998, and K. Annan at 212-759-0179. Ekua Annan, as she herself confirmed to the Sun, is Kofi Annan’s sister-in-law. The identity of the “K. Annan” is perhaps explained by a business card that Kofi Annan’s brother, Kobina, the diplomat now based in Morocco, was handing out in America five or six years ago. A copy, seen by this reporter, gives the same address and apartment number that Kojo Annan supplied to Cotecna in 1996 while his father was still living at the Roosevelt Island address, but it gives the Ekua Annan number for the business telephone, and the K. Annan number for a fax.

The “K. Annan” might also refer to Kobina and Ekua Annan’s son, Kobina Annan Jr., whose name appeared on March 9, 2005, in the New York Sun when reporter Benny Avni chronicled his work for a New York public relations firm, Ruder Finn, run by a friend of Kofi Annan, David Finn. In the thick of the Oil-for-Food scandal, Ruder Finn offered pro bono advice to Kofi Annan, while at the same time doing paid work for the United Nations.

The questions multiply. Did Kofi Annan ever give up the lease on his old Roosevelt Island apartment? If he did, how did it end up in the possession of his brother’s family? Why does the nephew appear to be the chief occupant today of an apartment that under Mitchell-Lama guidelines is meant for a low- to moderate-income New York family of between 5 and 6 people?

Mitchell-Lama regulations specify that in some cases a lease may be turned over to family members such as a brother or sister-in-law if they have lived continuously for two years previous to the handover in the same apartment, using it as a primary residence. Before moving out, did Kofi and Nane Annan share their Roosevelt Island apartment full-time between 1994 and 1996 with Kobina or Ekua Annan? If so, this information has not surfaced in interviews with either Kofi or Nane Annan to date.

Kofi Annan’s former dwelling is not the sole source of Roosevelt Island’s Mitchell-Lama mysteries. The enclave is known for its heavy concentration of U.N. staff, many of whom receive U.N. housing allowances on top of whatever savings they might enjoy at the expense of New York state taxpayers. One of Kofi Annan’s former special advisers, Italian businessman Giandomenico Picco, lived on Roosevelt Island in the late 1990s, and documents seen by this reporter, including a power of attorney, show him at 531 Main Street, another Mitchell-Lama building, until at least until 2001.

The questions about the Annan apartment are not the first about various Annan family ventures to have emerged only to go unanswered.

The secretary-general misled the press for years about the nature of his son’s business involvement with the Oil-for-Food program. When asked at a press conference last year about a Mercedes that his son shipped into Ghana in 1998 under false use of the secretary-general’s name and the U.N. seal, Mr. Annan ducked the question by accusing the reporter of being a bad journalist.

Ms. Rosett is a journalist-in-residence with the Foundation for Defense of Democracies.