EL PASO, Texas (AP) — More than 4,000 archaeological artifacts looted from Mexico and seized in the U.S. were returned to Mexican authorities on Thursday in what experts say is one of the largest repatriation ever made between the neighboring countries.

The items mostly date from before European explorers landed in North America and include items from hunter-gatherers in pre-Columbian northern Mexico, such as stones used to grind corn, statues, figurines and copper hatchets, said Pedro Sanchez, president of the National Archaeological Council of Mexico.

Seizures were made in El Paso, Phoenix, Chicago, Denver, San Diego and San Antonio by U.S. Immigration and Customs Enforcement agents, though most of the relics — including items traced to a 2008 theft of a museum in Mexico — turned up in Fort Stockton, a Texas town about 230 miles southeast of El Paso.

More than two dozen pieces of pottery were seized in Kalispell, Mont., where Homeland Security agents discovered that a consignor had paid Mexican Indians to loot items from burial sites deep in the Mexican Copper Canyon in Chihuahua, Mexico, authorities said.

Although most of the items turned over are arrowheads, several are of "incalculable archaeological value," Sanchez told The Associated Press. He said it was the biggest archaeological repatriation, in terms of the number of items, that the U.S has made to Mexico.

U.S. officials displayed the relics at the Mexican Consulate in El Paso before handing them over during a ceremony Thursday. The artifacts will eventually be taken to the National Institute of Anthropology and History in Mexico City, where they will be studied, cataloged and distributed to museums across Mexico.

Most of the items resulted from a string of seizures in West Texas in 2009, following a tip about relics illegally entering the U.S. at a border crossing in Presidio, Texas.

Homeland Security special agent Dennis Ulrich said authorities executing a search warrant in Fort Stockton found the largest portion of the cache. And further investigation revealed that the two men who organized the artifacts' smuggling were involved in drug trafficking from Mexico to the U.S., he said.

Sanchez said some of the relics found in Fort Stockton were stolen from a private collection at the Cuatro Cienagas museum in the Mexican state of Coahuila.

The items also include arrows, hunting bows and even extremely well conserved textile items such as sandals and pieces of baskets.