At least two police departments in Massachusetts are warning parents to inspect their children's Halloween candy for marijuana edibles.

But there has never been a recorded case of any one giving cannabis candy -- or razor blade apples, or poisoned Kit Kat bars -- to trick or treaters, according to fact checkers and a University of Delaware professor who studies Halloween poisoning myths.

Professor Joel Best has scoured news outlets for reports of dangerous Halloween candy since 1985, when rumors of malicious adults handing out poisoned chocolate bars and razor blade-spiked apples had already sparked a minor national panic.

What he found is that every reported case was either a hoax, or in one tragic case, a father deliberately killing his own child and using the rumor as a cover story.

"The bottom line is I have never ever found a report of a child killed or seriously hurt by a contaminated treat," Best said. "There have been some reports but they have always been retracted."

And with the increasing prevalence of cannabis edibles, which contain high concentrations of THC and can look like ordinary candies, parents and police departments have found a new vehicle for concern, according to Best. Cannabis edibles can cost $10 per piece, Best noted, making it unlikely that owners would distribute them at random.

"I was kind of nervous about that in the first year of [legalization] but I have been unable to find even a report of this happening," Best said. "They imagine bad things that could happen, and when a new bad thing comes along they add it to the list."

The fact checking website Snopes has debunked the claim of people deliberately distributing poisoned Halloween candy.

This year in Massachusetts, Walpole's police chief tweeted a warning to parents about the risk of finding cannabis in their children's candy stash.

Parents:This year please be bit more cautious inspecting Halloween unwrapped/unmarked candy for THC infused products pic.twitter.com/BB0vSflCs0 — Chief Carmichael (@WalpoleChief) October 27, 2017

And the Springfield Police Department posted on Facebook, telling parents to be aware of the risk.

A message from the Springfield Police Department this Halloween: Parents should be aware of children accidentally... Posted by Springfield Police Department on Tuesday, October 31, 2017

In an interview, Springfield Police spokesman Ryan Walsh said the department had no specific knowledge of cannabis candies being given out on Halloween. But, he said, other types of accidental exposure are a real threat, citing the case of a 10-year-old boy in Ramapo, New York who was hospitalized after a cannabis gummy.

"We haven't seen it on Halloween, but as more and more edibles get into our society and in Massachusetts the likelihood, as small as it may be, increases," Walsh said. "It's just really an exposure issue on this day - an extra heads up. Nothing we're looking at specifically."

Walsh's concern is backed up by research. In Colorado, which legalized recrational marijuana in 2012, child cannabis exposures increased 34 percent per year from 2009 to 2015, with edibles accounting for more than half of the exposures, according to a study in JAMA Pediatrics. Incidents remained rare overall, with a total of 163 reported from 2009 to 2015, though Colorado's increase outpaced the national average.

Springfield's warning came after the department received a briefing from the New Jersey State Police Regional Operations and Intelligence Center, Walsh said. Both Pennsylvania and New Jersey authorities issued similar warnings, citing the same Ramapo incident.

Jim Borghesani, the Massachusetts spokesman for the pro-cannabis Marijuana Policy Project, said he recognized that cannabis edibles can be dangerous for children, but argued they should be treated like any other risky substance.

"Parental responsibility is necessary for any substance, including marijuana, alcohol, detergent," Borghesani said in an interview. "You can't legislate common sense, but you would hope that adults take the proper precautions when it comes to exposing their children to any dangerous substance."

And he suggested that police departments are unfairly targeting cannabis by singling the drug out for criticism on Halloween.

"No, I have not heard of this happening in Massachusetts and I have not heard of this happening in any of the states that have legalized before Massachusetts," Borghesani said. "I think this is a scare tactic. I think there would be more credibility if these police chiefs also tweeted about looking for alcohol products and tobacco products and any other drug."