An American antiquities collector has returned a dozen looted ancient artifacts, including decorated pottery and bronze jewelry, to Thailand, which has been campaigning in recent years for the return of smuggled treasures.

The objects are believed to be between 1,800 and 4,300 years old and made by an ancient civilization in Ban Chiang, a UNESCO World Heritage Site in northeast Thailand. Thai culture minister Vira Rojpojchanarat announced the return of the prehistoric artifacts on Thursday, the Associated Press reported.

It remains unclear how the collector, Katherine Ayers-Mannix, obtained the antiquities, which she turned over to the Thai Embassy in Washington, DC.

In 2014, the US government returned 554 ancient artifacts from Ban Chiang to Thailand after they were discovered in a 2008 raid on the Bowers Museum in Santa Ana, California. That incident was believed to be part of a larger Southeast Asian smuggling network.

The Thai government is currently investigating objects at several US museums. In February it called for the return of looted cultural patrimony in institutional collections. Nine ancient Buddhist relics displayed at the Norton Simon Museum in California—including seven Buddha statues carved over a millennium ago—and 14 artifacts on view at the Honolulu Museum of Art are said to have been taken illegally from the country. The Asian Art Museum in San Francisco has also removed two ancient lintels from display after finding evidence that they were stolen from temples in northeastern Thailand.

The Thai government has passed along its findings to the US Department of Homeland Security, hoping to secure the artifacts’ repatriation. The recently recovered items will go on display at the National Museum in Thailand.