Jacqueline Kennedy Onassis is the purchaser of one of the most important open tracts of land on the Vineyard, specifically 375 acres of strategically placed property in the Squibnocket Pond area of Gay Head.

The Gazette confirmed Mrs. Onassis' ownership after a widespread check of sources in Washington, New York and elsewhere.

Until now the acquisition of this land, variously known as the Hornblower property and the Red Gate Farm, has been clouded in secrecy.

The Gazette last January reported the $1,110,000 sale of the land to Alexander D. Forger, a member of the New York law firm of Milbank, Tweed, Hadley and McCloy. Mr. Forger has conceded he bought the land in trust, but he has refused to identify the individual behind the purchase.

In recent weeks rumors have pointed toward the involvement of Mrs. Onassis in that large but fragile piece of Gay Head land. But the reports never reached beyond the rumor stage.

Mrs. Onassis, vacationing at her Cape Cod home in the old Kennedy compound at Hyannis port, failed to respond to phone calls placed to her, by the Gazette.

Signs that Mrs. Onassis had purchased the land and apparently intends to build in Gay Head have accumulated on the Island over the past three weeks. There were these confirmed reports:

• Mrs. Onassis has been on the Vineyard more frequently this summer than at any time in recent years.
• This month she toured her new property with a person reported to be one of her financial advisers.
• During a recent but different trip to the Vineyard Mrs. Onassis was accompanied by an architect, the Gazette learned. With the architect, she is reported to have made a careful study of at least one famous house on the Island, presumably to get building ideas of her own.
• On her most recent sailing trip to the Vineyard, her boat was moored off Menemsha, the harbor nearest to the Gay Head property which includes all of the western shore of Squibnocket Pond and four-fifths of a mile of ocean beach. On that particular trip Mrs. Onassis dined one night at the Home Port restaurant in Menemsha after touring her property.
• Mrs. Onassis had conversations while on the Island about her future plans for the sweeping stretch of Gay Head land.
• Private negotiations for land on the Vineyard, conducted through her attorney Mr. Forger, have always stressed the need for absolute privacy and isolation of the location. Other land was considered before the final January purchase of the Gay Head tract.

Sources who appeared to know something about the sale generally refused to tie Mrs. Onassis specifically to the property. But these sources also indicated they were satisfied that the land will not be used improperly and that it will remain in its natural state and carefully conserved.

When the land was sold to Mrs. Onassis in January, Land Vest Inc., a Boston-based real estate firm, released a statement saying the unnamed buyer is "dedicated to the Cape and Islands and their conservation."

The plan, Land Vest said, is to "keep the area under the conditions it has enjoyed, for as long as it is possible to envisage. One house may be built within the next five years, and perhaps another in 20 years.

"The moors and dunes, the deer and the birds, will remain as they are, undisturbed by development."

The president of Land Vest, Richard F. Perkins, offered this additional comment at the time of the property sale to Mrs. Onassis:

"The region has recently been the subject of some local concern because of the beauty and sensitivity of its dunes and marshlands. Land Vest is known for its careful approach to land use and therefore was retained by the Horn-blower family to manage and land plan the property, and has been working for the past several years to determine its best use and find a buyer who would protect this special place."

Mr. Perkins also spoke of an apparent commitment to the Hornblower family: "The Hornblower family is satisfied that the buyers' interest is in keeping with the continuance of the conditions maintained in the past, and is pleased to make the sale."

Mr. Forger Thursday refused either to confirm or to deny the Gazette story. "I am holding the land in trust," he said.

But he did speak out more strongly than before about what is intended in future planning for that area of the Island. "The buyer is more determined than ever that the land be preserved and that it be so kept," Mr. Forger said.

"This is a unique parcel of land in an ideal setting, and it will be kept that way so far as can be determined in the future. If there are concerns and apprehensions in the Vineyard community, they ought to be allayed.

"There is no question of development here. The land is as safe as anyone could imagine.

"There is no intention of disposing of the land, and it will not be organized with any development in mind."

Mr. Forger said he simply could not go beyond these comments and that he was in no position to talk about the specific buyer. He said, moreover, that by his silence he did not mean to acquiese in the Gazette story.

The January sale of the Gay Head package to Mrs. Onassis was arranged through Land Vest, acting with David Flanders of Chilmark.

The Onassis property lies between the Moshup Trail and Squibnocket Pond. The only building on the property at present is a modest shingled house overlooking the Pond.

Concern for the future of this piece of land and that area of the Vineyard generally dates back more than a decade. In 1965, for example, Richard H. Pough, seasonal resident of Chilmark and noted conservationist, warned of the urgent need to preserve the area.

"Squibnocket Pond," he said, "is the only one of the Vineyard's large shallow ponds that is still fresh and a haven for waterfowl. It still supports a herring run and is bordered by the most beautiful complex of dunes, small ponds, and orchid filled lowlands to be found in New England."