A sticky red stain that looks like blood first appeared outside the downtown Tempe post office on Halloween night, postal officials said Friday.
But nobody seems to know if the substance is actually blood or something else.
U.S. Postal Service spokesman Peter Hass said vandals spread the red substance on post office windows and elsewhere on the property on Oct. 31. He said postal workers attempted to clean the gruesome scene the next morning but were unable to remove all the mess.
He said postal workers would try again on Friday to remove the red stain that pedestrians have been stepping through and around since Halloween.
"They are out there again today attempting to clean up whatever else remains," Hass said.
He said the vandalism was not reported to Tempe police because the incident happened on federal property. Instead, he said the U.S. Postal Inspector's Office handled the investigation.
Postal inspector Patricia Armstrong in Phoenix said the vandalism was reported to her office after the cleanup had already occurred.
"We did not have a crime scene to investigate," she said.
Armstrong said the nature of the red substance will likely remain a mystery.