Federal agents raided several Southern California museums on Thursday in search of looted antiquities, as part of a five-year undercover investigation of alleged smuggling operations.

Federal agents raided the Los Angeles County Museum of Art, the Bowers Museum in Santa Ana, Pasadena’s Pacific Asia Museum and the Mingei International Museum in San Diego on Thursday, the Associated Press quoted Virginia Kice, a spokeswoman with Immigration and Customs Enforcement, as saying. Agents also investigated American Indian artifacts at one museum.

Although no arrests were made and no charges were filed, authorities believe the smuggling of artifacts is actually being conducted at a grander scale than previously exposed.

The modus operandi appears to the following: the owner of a Los Angeles art gallery collaborated with a smuggler who brought artifacts from Thailand and China; they were offered as charitable contributions; the donations would then be claimed as tax write-offs, the AP reports.

While some museum officials sometimes questioned how the objects had been obtained, they eventually accepted them, which makes their involvement dubious.

Michael Govan, director and chief executive officer of the Los Angeles County Museum of Art, was quoted by the wire agency as estimating some 60 items donated to the museum over the past decade have come under suspicion. Govan added the museum was cooperating fully with the investigation, as are the other institutions.

The search warrants served Thursday are a result of a five-year undercover investigation by an unnamed National Park Service special agent who presented himself to the case’s two main suspects as an avid art collector.

The agent targeted Robert Olson, an alleged art smuggler, and Jonathan Markell, who owns a Los Angeles Asian art gallery; his art gallery was raided as well, the Los Angeles Times specifies.

Olson and Markell told the undercover agent over the years how they obtained artifacts smuggled from Thailand, China and Burma and then sold them to clients in Los Angeles. They had also laid out a scheme to obtain fraudulent tax deductions.

They sold looted artifacts worth a few hundred dollars with forged appraisals that inflated the objects’ value to just less than $5,000; they then helped their clients donate the objects to local museums and thus obtain tax deductions.

The Internal Revenue Service requires documentation on donations of objects with values that do not exceed a $5,000 limit.

The grimmest part of the investigation is that museum officials appeared to be aware of the illegal nature of the donations and went along anyway.