A new drug called "grey death" has been linked to a handful of lethal overdoses in the South -- but no one knows exactly what's in it or where it's coming from. The drug, a mix of opioids, can kill in very small doses and looks like concrete mixing powder. Over the past four months, Georgia police have seized about 50 batches of grey death statewide, with metro Atlanta being a major hotspot. The drug has also started popping up in Alabama, Ohio and Pennsylvania, state and local officials confirmed.

Because ingredients change from sample to sample, each batch of grey death is a mystery -- right down to its signature color. Investigators have found a variety of opioids in grey death samples: designer drug U-47700, heroin, fentanyl and fentanyl-like molecules. Because each drug is present in such low concentrations, some of them may not show up on tests, Kilcrease said.

"There have been several overdoses and overdose-related deaths across Georgia and Alabama linked to grey death," a report from the Gulf Coast High Intensity Drug Trafficking Area said. Toxicology and autopsy results are still pending for the Georgia cases, Kilcrease said.