Thanks to a group of eagle-eyed scholars, a trove of stolen colonial-era documents has been returned to Mexico City.

Unidentified thieves smuggled the 16th-century papers out of Mexico’s Archivo General de la Nación (AGN) in a “systematic,” multi-year operation, reports Raúl Cortés Fernández for Reuters. Researchers first raised concerns about the possible heist after noticing some of the archive’s documents inexplicably appearing at auction in 2017.

Most of the manuscripts have direct links to Hernán Cortés, leader of the Spanish forces that invaded the Aztec capital of Tenochtitlán, waged war against the region’s Indigenous peoples and launched the colonization of modern-day Mexico.

American authorities returned the documents in a ceremony held at the Mexican consulate in New York City last Thursday, per a statement posted on Facebook. As Adyr Corral reports for Mexican newspaper Milenio, the cache of stolen goods includes 15 handwritten papers, as well as a small collection of looted antiquities that the Manhattan District Attorney’s Office recovered over the course of its investigation.

“This is one of the most important recoveries of documents in the history of Mexico,” said Mexican foreign affairs minister Marcelo Ebrard at the Thursday event, per Milenio (as translated via Google Translate).

The pages were cut from their original binding, smuggled out of the archive and illegally sold through such major auction houses as Christie’s and Bonhams. They garnered tens of thousands of dollars at auction, notes Rosa Vilchis for Noticieros Televisa.

Academics initially raised questions about the auctioned items’ provenance in 2017. The lots—royal decrees, legal records and even a rare letter bearing Cortés’ own signature—looked suspiciously similar to documents that were supposed to be held in the Mexico City archive, as Drazen Jorgic and Cortés reported for Reuters in May.