ATHENS, Greece (AP) _ Overseers of the Onassis fortune on Monday dismissed allegations of a plot to kidnap the estate's 12-year-old heiress, but acknowledged they did hire investigators to look into the girl's security.

The statement _ coming amid reports of a plan to abduct Athena Roussel, the granddaughter of late Greek shipping magnate Aristotle Onassis _ was the first public acknowledgment by the four trustees that they initiated a private investigation into her care.

They said the kidnapping reports were ``ridiculous stories invented'' by the girl's father, Thierry Roussel, who is feuding with the Onassis Foundation over control of and estimated $2.4 billion inheritance. Her mother _ Onassis' daughter, Christina _ died in 1988.

The statement said the trustees filed a detailed memorandum with Swiss authorities investigating the alleged plan to seize the girl last February in the ski resort of St. Moritz. Roussel lives with Athena, his Swedish wife and their three children in Switzerland and France.

An Israeli, reportedly a former member of the Israeli secret service, was arrested by Italian police last week on a Swiss warrant for allegedly planning the kidnapping. The investigating judge also suspects six other Israelis may have been involved.

The Israeli government has said that former secret service agents were hired by the trustees to carry out surveillance on Athena.

The trustees's two-page statement backed Israeli versions, but gave few details.

``The whole story was a security check which we ... had the right and have been forced to perform ... in order to protect (Athena) for a good many lawful and valid reasons,'' the statement said.

The trustees could not be immediately reached for elaboration, including who was hired to carry out the investigation.