The Philadelphia Police Department is investigating the source of a package containing two preserved fetuses in glass jars sent to the city’s medical and science Mütter Museum.

The museum staff reported receiving the package Tuesday morning, police said in a statement.

The package was addressed to the museum curator with no return address and contained a letter from someone claiming to be a retired physician stating the two specimens were a donation to the museum.

“There’s no type of proper paperwork, provenance or information about it that would enable us to accept that,” Anna Dhody, the Mütter Museum curator, told NBC Philadelphia. “Obviously, because they appeared to be human remains, we had to call the authorities.”

The Mütter Museum is part of the College of Physicians of Philadelphia and displays preserved collections of anatomical specimens, models and medical instruments.

The museum does accept donations, but the process, especially for anything involving human remains, takes research and requires a detailed account of the object’s history.

Dhody has been the museum curator for almost 20 years and says the package donation was “atypical, abnormal and very much not the procedure.” She told NBC News she discourages others from sending such “unsolicited, anonymous remains.”

The fetuses have been turned over to the Medical Examiner’s Office for further investigation. The Philadelphia Police Department will provide additional information as it becomes available.