It was a “sophisticated method of laundering,” a prosecution witness testified on Friday in a trial courtroom here. Private collectors would acquire looted artifacts and eventually donate them to museums, she said.

The witness, Daniela Rizzo, an Italian archaeologist, specifically cited the owners of four American antiquities collections.

None are on trial here. None have been legally charged with any wrongdoing. Nor do Italian prosecutors contend that the collectors had evidence that certain objects had been looted. Yet the prosecutors have clearly adopted a strategy of calling attention to collectors, especially well-heeled Americans, with the implicit message that every player in the global antiquities trade is within their sights.

The actual trial defendants are Marion True, a former antiquities curator at the J. Paul Getty Museum in Los Angeles, and Robert Hecht, an American dealer in classical artifacts. Both are charged with trafficking in objects that were illicitly excavated from Italian soil.

Dozens of witnesses have filed through the courtroom since the trial opened in November 2005, but only this month did prosecutors fully turn their sights on American antiquities collectors in court testimony. The tactic has infuriated defense lawyers, whose objections became so heated on Friday that Judge Gustavo Barbalinardo decided to suspend the proceedings until tempers cooled.

In her testimony Ms. Rizzo said that the “very rich” private collectors built collections that “included objects that were undoubtedly looted from Italy.”

She cited the Texas oilmen Nelson Bunker Hunt and William Herbert Hunt (who sold their artifacts at auction in 1990 after their fortunes collapsed); the New York diamond merchant Maurice Tempelsman; the art philanthropist Lawrence Fleischman and his wife, Barbara; and the financier Leon Levy and his wife, Shelby White.

Objects from their collections went on display in major exhibitions, “becoming known to the public and the scientific world, after which they ended up in museums,” Ms. Rizzo said. “It was a different, more sophisticated method of laundering artifacts.”

Defense lawyers for Ms. True and Mr. Hecht repeatedly interrupted Ms. Rizzo’s testimony; Francesco Isolabella, a lawyer for Ms. True, objected that it was beyond Ms. Rizzo’s mandate to “come up with inductive or deductive theories.” Mr. Isolabella said that the archaeologist was making “evaluations that only a prosecutor can make.”

“She should stick to identifying Etruscan vases,” Mr. Isolabella said.

Because they are not on trial, none of the collectors are represented in the courtroom. Contacted through their offices, Nelson Bunker Hunt and Mr. Tempelsman declined to comment on the trial or to be interviewed. Through a spokesman, Ms. White declined to comment on the trial. (Her husband died in 2003.) In a telephone interview from New York, Ms. Fleischman said: “It seems like anyone can accuse anyone of anything without any proof. We collected for the pure joy of the object.” She declined to comment further on the trial. But she said that she and her husband, who died in 1997, never suspected that they might be buying anything less than legitimate.

Ms. Fleishman, a former Getty trustee, and her husband donated or sold more than 300 antiquities to the Getty. Italy’s Culture Ministry is demanding that the museum return a dozen of the Fleischman objects.

It seems inevitable that as prosecutors began investigating the passage of objects from looted tombs in the Italian countryside to American institutions, they would become interested in private donors, long the lifeblood of the world’s museums.

A report this year by the Association of Art Museum Directors, which represents nearly 200 museums in the United States, Canada and Mexico, notes that “more than 90 percent of the art collections held in public trust by American’s art museums were donated by private individuals.”

Beyond altruism, the economic benefit for collectors in the United States is a tax deduction equal to the current market value of the object. This deduction can save the donor 25 to 50 percent of the object’s value, depending on what method is used. In return, museums short of funds can aspire to top-rate works of art.

“There’s every reason to think that a museum would bring an item it can’t afford to the attention of a collector, hoping for a donation,” said Patty Gerstenblith, a law professor at DePaul University and president of the Lawyers’ Committee for Cultural Heritage Preservation.

At the same time, she said, “it helps to distance the museum from a questionable piece.” There is less risk of public disapproval should a museum be forced to return a piece that arrived as a donation, she said.

For at least the past decade, though, Peter C. Marzio, director of the Museum of Fine Arts in Houston and a past president of the Association of Art Museum Directors, said, both collectors and museums have been conducting “diligent provenance inquiries” on artifacts that change hands.

He cited an auction on Thursday at Sotheby’s in New York where a 2,000-year-old bronze statue of Artemis sold for $28.6 million, a world record for sculpture and for an antiquity.

“Provenance is what is driving prices up,” Mr. Marzio said. “Provenance is having enormous value.” The Albright-Knox Art Gallery in Buffalo acquired the Artemis from a Manhattan dealer in 1953, well before the adoption of a 1970 Unesco convention governing traffic in cultural property.

During a courtroom break, Mr. Hecht, the dealer on trial here, scoffed at the notion of a collector like Ms. White having an economic incentive for donating to a museum. “The last thing Shelby Levy White needs is a tax deduction,” Mr. Hecht said. She “really liked and learned about every piece she bought.”

Running parallel to the trial is the Italian Culture Ministry’s dogged effort to lay claim to what it contends are looted objects in American museums. Last year it negotiated the return of 21 antiquities from the Metropolitan Museum of Art and 13 from the Museum of Fine Arts, Boston.

Talks with the Getty have faltered. Italy is seeking the return of 52 objects over all from that museum, among them a dozen donated by Ms. Fleischman. So far the Getty has agreed to return 26 artifacts to Italy, including 5 acquired from the Fleischmans and three pieces purchased from Mr. Tempelsman.

In an interview Paolo Ferri, the trial prosecutor, asserted that American collectors might one day find themselves at the defense table.

Once a verdict is delivered in the trial of Ms. True and Mr. Hecht, he said, “I’ll draw my conclusions.”