Mesoamerican archaeologists know it as Monument 9: a 2,600-year-old carving in stone of a jaguar’s gaping face, roughly five feet wide and tall and weighing one ton. Nearly 60 years ago the relic was looted from the ruins of Chalcatzingo, an Olmec site south of modern-day Mexico City, and smuggled into the United States, where it disappeared into a network of private collections.

The absence of the relic, constructed between 700 B.C. and 500 B.C., long vexed Mexican scholars. In its time the stone would have been used as a portal for priests and rulers to pass into the underworld, but the few photos that existed of Monument 9 could not fully convey its symbolic heft.

In March, however, U.S. authorities notified Mexican officials that they had seized the stone after tracking it to a warehouse in Denver. And in May the carving returned home in style, escorted by military vehicles from the airport in Cuernavaca, Mexico, to a nearby regional museum.

“Having it here on hand is like placing the last piece of the puzzle and starting to see how it worked,” Carolina Meza, the chief archaeologist at Chalcatzingo, said. “It’s unique at a Mexican, Mesoamerican and global level.”

In recent years Mexico has mounted an ambitious series of investigations and restitution efforts to reclaim the nation’s stolen cultural heritage, joining with other countries to correct decades of theft and colonial plundering. Monument 9 might be Mexico’s biggest prize yet — for researchers, for the communities who still practice elements of Indigenous culture and for sheer archaeological drama.

The stone carving offers a mix of familiar imagery and remarkable practicality. Features, including distinctive thick eyebrows and bromeliad plants that extend from the corners of the jaw, date the piece to the Olmecs, who settled in the region after moving inland from the Gulf Coast 2,800 years ago. At its center, representing the animal’s mouth, the stone opens in a quatrefoil cavity that “fits a person perfectly,” Ms. Meza said.

“It is as if it were a door that is transiting between dimensions other than the one in which the human being lives as a human,” she added.

For the Olmec, the underworld was considered the birthplace of humanity and the home of the soul, a mystical plane bearing little resemblance to the fiery landscape of biblical narratives. At Chalcatzingo, a sprawling site situated between a pair of rocky outcrops, the piece may have been fitted over the entrance to a cave or building and used in ceremonies related to coming-of-age or a transition into the priesthood.

Mexico’s restitution campaign — “Mi Patrimonio No Se Vende,” or “My Heritage Is Not for Sale” — taps into a changing moral consensus around the ownership of antiquities, which has manifest in numerous public controversies, notably the protracted negotiations between Britain and Greece over the Elgin Marbles at the British Museum., which were taken from the Parthenon two centuries ago.

“It’s an awareness or shame that’s generated in someone who has archaeological pieces from Mexico or other countries on display when someone visits their home,” Alejandra Frausto, Mexico’s culture secretary, said in an interview.

Since the campaign began in 2019, Mexico has won back more than 13,000 artifacts, often celebrating the recoveries in press events held before banks of cameras. In September, the San Bernardino County Museum in California announced the return of nearly 1,300 small objects, including pre-Hispanic jewelry and wind instruments.

One successful tactic has involved waiting for Mexican relics to come up for auction overseas, at which point Mexican authorities pounce, filing letters of protest and “making a lot of noise,” Ms. Frausto said. “There’s nothing more vulgar or cheap than putting a price on a symbol of our identity.”

But many artifacts remain beyond reach. A brilliantly feathered headdress, believed by some scholars to have belonged to the 16th-century Aztec ruler Montezuma, is on display in a Vienna museum. In 2020, Beatriz Gutiérrez Müller, the wife of Mexico’s president, Andrés Manuel López Obrador, made a request in person to the Austrian government to repatriate the piece, but the government rebuffed her, arguing that the piece was too fragile to move. Mexico has disputed that assessment.

Mexican authorities have also been working international legal channels to track down and reclaim relics that aren’t offered up voluntarily.

Last year, responding to a request from the Mexican consul general in New York, investigators from the Antiquities Trafficking Unit in the Manhattan District Attorney’s Office opened a probe into the location and handling of the missing Monument 9. Each of the 19 members of the squad are part investigative journalist, part Indiana Jones, armed with the power of the subpoena. They determined that the piece passed through New York at some point, giving them jurisdiction to investigate the item’s theft.

“We always investigate in two directions,” Matthew Bogdanos, the head of the unit, said. “We investigate now, backwards, and then we try to find the looting site and move forward, whether it’s a museum, a gallery, a villa, a valley in Egypt or, in this case, a valley in Mexico.”

In Chalcatzingo, investigators identified key witnesses to the stone’s final moments in Mexico: two farmers who recalled seeing the piece as children. The interviews revealed that the relic was initially discovered in 1962 by workers tilling a field, and that by 1964 it was broken in pieces. That year, Mr. Bogdanos said, “a bunch of gringos — foreigners — came and wrapped the pieces in large leaves in order to protect them and then put them in the back of a truck and took it away.”

From there, investigators believe, the Olmec stone crossed into the United States hidden in the shipments of a well-known looter, William Spratling. In 1965 it surfaced in New York, photographed in an advertisement that produced a sale for $2,000.

The relic traded hands four more times, appearing briefly in public at museum shows, including at the Metropolitan Museum of Art, which recorded the item in a 1970 exhibition book as a “colossal jaguar mask” from the collection of the Munson-Williams-Proctor Arts Institute in Utica, N.Y.

Working backward from the present, the New York team found a recent listing for the piece at an auction house. Its information about the seller pointed to a warehouse in Denver, where the owner stored the stone after purchasing it in 2000 for $2.25 million. When authorities seized the relic in March, it was being offered for sale for $12 million.

No charges have been filed in the case, although an investigation into grand larceny and fraud, among other crimes, is ongoing, Mr. Bogdanos said. Officials have not publicly named the piece’s final collector, in line with a practice observed when the owner appears to not have been aware of the item’s original theft.

Officials in Mexico plan to celebrate the success of the restitution campaign by bringing together many of the recovered artifacts in an exhibit next year, Ms. Frausto, the culture secretary, said. Eventually, the Olmec stone will return to Chalcatzingo, where a new museum is being built.

For now the Olmec stone sits illuminated on a pedestal in the entrance hall of the Museo Regional de los Pueblos de Morelos in Cuernavaca, an hour away from the archaeological zone, and draws long lines for viewings, Rodolfo Candelas, the museum director, said.

He recalled that, soon after the piece was unveiled, visitors from Chalcatzingo laid an offering of fruit in front of the stone as a gesture of welcome.

“That is the importance: These pieces still have significance,” Mr. Candelas said. “They still talk to you, they still tell you something. They remind you of what was. They remind you maybe a little of what there is.”