Philadelphia ? It’s that time of year when even cemetery guards are more easily spooked, on alert for signs of one of the oldest, most ghoulish and most persistent of crimes: grave robberies.

Mostly, it’s kids doing pranks or thieves looking for valuables, metals or marble. Occasionally, it’s the work of occultists looking for ritual objects. But while such grave thefts remain rare, the potential for holiday trickery always puts cemetery officials on high alert.

“It’s more of a preventative thing,” said Bob Fells, general counsel for the International Cemetery Cremation and Funeral Association. “People are going to say, ‘Well, didn’t you take any steps, since it is Halloween?”‘

Guards keep a closer eye on their rounds, and security workers are extra vigilant when monitoring motion sensors and cameras. There’s no definitive evidence that thefts go up around Halloween, but the extra effort is not without reason.

Back in June, police in Philadelphia arrested two teenagers for stealing remains from a 15-foot underground crypt, possibly to be sold for use in an occult ritual. That same month, two other teens were accused of stealing a human skull from a tomb in Hernando County, Fla.

And also in Florida, detectives exhumed the remains of a woman in a St. Petersburg cemetery Thursday to determine whether a skull found in a former gravedigger’s bedroom was hers. It was.

It isn’t always as extreme as body parts, though.

In recent years, thieves have started going after bronze artifacts as the price of the metal has skyrocketed, Fells said. In response, some cemeteries have made changes, including hiring private security and started locking up more often. In some areas, police started conducting more frequent patrols.

Dan Rohling, a crematory operator and funeral director in California, said vigilance is always higher this time of year. The theft of valuables and body parts from graves and grave sites, he said, is the work of “morons” who just aren’t thinking.

“They know not what they do, and God knows why they do it,” Rohling said, adding that grave robberies remain a persistent but rare problem. “It hasn’t really gone up. It hasn’t really gone down. You have the same underlying group of morons out there.”

The crime itself dates at least back to thefts from Egyptian tombs.

Hundred of years later, in the 19th century, there was a spike as medical experts found use for studying the human body. Because organ donation programs hadn’t been developed, doctors ended up having to steal bodies for research.