An Irish museum recently received a strange parcel: two 4,000-year-old ax heads, carefully packed in a repurposed flapjack box and mailed by an anonymous donor.

An unsigned note in the package said the artifacts were discovered with a metal detector in central Ireland, according to a statement from the National Museum of Ireland (NMI), which received the axes in late June. The sender, who “expressed a desire for the axeheads to be conserved,” included no further details and provided no contact information.

“The ax heads were thoughtfully packed in foam cut-outs and cardboard, ensuring their safe arrival,” says the museum. “Our experts at the NMI have identified these items as flat ax heads from the early Bronze Age, a significant archaeological find that offers a glimpse into Ireland’s distant past.”

Now, the museum is asking the sender to come forward. Officials hope to learn more about exactly where the ax heads were unearthed.

“We are thrilled about the discovery of these early Bronze Age ax heads, but to truly understand their significance, we need to know where they were found,” says Matt Seaver, the museum’s assistant keeper of Irish antiquities, in the statement.

The flat metal tools date to between 2150 and 2000 B.C.E., a few hundred years after the early Bronze Age began in Ireland. The era is named for innovative metalworking techniques that swept the region, including the forging of bronze—a durable mixture of copper and tin.

The museum is currently participating in an international study of Bronze Age metalwork, and the newly acquired ax heads could provide crucial data, reports Laura Fletcher for the Irish news service RTÉ News.

The “beautiful” ax heads were “among the earliest metal axes to have been used in Ireland,” Seaver tells RTÉ News. The metals they were forged from may have “come from somewhere like the copper mine in Ross Island”—Ireland’s first copper mine, located in the southwestern county of Kerry.

The axes’ precise geographical context could also shed light on the “ancient settlement patterns and cultural practices” of the people who used them millennia ago, according to the museum. “For instance, hoards or collections of objects were often deliberately placed in specific locations for reasons that could range from ritualistic to supernatural.”

Museum officials think they know why the sender wanted to stay anonymous: As Seaver tells RTÉ News, the donor was likely “aware of the law around the restrictions on using metal detectors to search for archaeological objects.”

In Ireland, metal detectorists need written permission from the government to legally hunt for treasures, per BBC News’ Eimear Flanagan. The same restriction may have been at play in a similar case in 2016, when another anonymous treasure hunter sent a box of Bronze Age axes and Viking jewelry to the museum.

Similar mysteries have also occurred outside of Ireland. Earlier this year, Poland’s Royal Łazienki Museum received a mysterious package containing a dozen 17th-century tiles that once decorated a Baroque bathing pavilion in Warsaw.

Irish museum officials insist that the recent donor would not be penalized for coming forward. Any volunteered information about the axes “would be treated in confidence and would only be used to fill out the discovery of the object,” Seaver tells RTÉ News.

“An object on its own is valuable in one sense, but an object in its context tells us something about where and why these objects were used,” he adds. “Things that gave these objects meaning were where they were placed in the landscape.”