Coordinated raids on four Southern California museums early Thursday suggest that the involvement of art institutions in the purchase of looted objects is far more extensive than recent high-profile scandals have indicated.

Even as the country's most prominent museums were embarrassed by revelations of stolen artifacts in their collections, several local museums continued to pursue objects they had reason to believe were taken illegally from Thailand, Myanmar, China and Native American sites within the United States, according to search warrants served Thursday.

Dozens of federal agents descended on the Los Angeles County Museum of Art, Pasadena's Pacific Asia Museum, the Bowers Museum in Santa Ana and Mingei International Museum in San Diego.

The raids marked the first public move in a five-year undercover investigation of the alleged smuggling pipeline.

Museum officials defended their practices and pledged to cooperate fully in the investigations.

The detailed warrants gave the agents broad authority to search the museums' galleries, offices, storage areas and computer archives. They were looking for objects and records related to the primary targets of the investigation: an alleged art smuggler, Robert Olson, and the owner of a Los Angeles Asian art gallery, Jonathan Markell. Markell's Silk Roads Gallery on La Brea Avenue was also raided.

No arrests were made, but legal experts say the surprise search warrants suggest prosecutors are collecting the final elements to seek criminal indictments against Markell and Olson.

The action comes after several years in which the art world has been hammered by claims from Italy and Greece that major American museums -- most prominently the J. Paul Getty Museum -- purchased art that had been stolen from and smuggled out of those countries.

The Getty agreed last year to return 40 of its most prized objects, following similar deals by museums in Boston and New York. The Getty's former antiquities curator is on trial in Rome, accused of knowingly buying looted art, a charge she denies.

This case could go further. The warrants served Thursday show prosectors have carefully laid a foundation for the possible indictment of museum staffers allegedly complicit in the looting schemes -- which would be a first under American law, experts say.

The warrants are based on a five-year undercover investigation by an unnamed agent with the National Park Service, who presented himself to Olson and Markell as an eager collector.

Olson and Markell told the undercover investigator they regularly bought Thai antiquities from looters and smugglers, sometimes smuggling them personally, the warrants state. They then allegedly sold them to clients in Los Angeles. They also admitted running an elaborate donation scheme, selling their clients looted artifacts with forged appraisals that inflated the value of the objects by as much as 400%, the warrants state. They then allegedly helped these clients donate the objects to local museums, which provided a tax write off at the inflated value.

The scheme appears to have spanned the last decade and generally involved repeated donations of objects with values of just under $5,000, the value at which the IRS required additional documentation.

In the case of the Bowers and the Pacific Asia Museum, the warrants clearly suggest that officials were aware that the objects were looted and overvalued but accepted them anyway.

A senior curator at the Bowers Museum, now deceased, regularly accepted donations of Thai and Native American antiquities from Olson that he knew were removed illegally, the warrants say. The documents describe the longtime curator, Armand Labbe, smiling and chuckling as he told the agent he could accept the donations because "he could not determine" what rules the museum was supposed to follow.

The Bowers' current director, Peter Keller, told the undercover agent he knew Olson and had visited his warehouse. An appraiser, described as Labbe's girlfriend, told the agent that she regularly prepared appraisals of objects for Keller to donate to his own museum. Keller denied wrongdoing Thursday.

In the case of the Pacific Asia Museum in Pasadena, the agent met with two museum officials in in November 2005 and offered to donate Thai material he had purchased from Markell.

Marcia Page, the museum's deputy director of collections, told the agent that given her position, she was "expected to at least put up token resistance to accepting antiquities without proper paperwork," the warrant states. She noted that Markell had donated Thai material, and that she didn't think it was legal. She said more senior people in the museum would have to sign the paperwork.

About two weeks later, Page told the agent that the bulk of his donation had been accepted.