While other death penalty states have struggled to get their hands on execution drugs, the state of Missouri has quietly stockpiled a substantial amount.

It raises the question: How is Missouri able to do this when no other state seems to have a steady source of the drugs? The state isn't saying, at least not anymore.

The state will also no longer say whether officials procure execution drugs from a compounding pharmacy, where pharmacists mix the drugs most often used in executions.

"Missouri uses pentobarbital as the lethal chemical in its execution process, but does not admit nor deny the chemical now used is compounded as opposed to manufactured," Attorney General Chris Koster's Office wrote in an April court filing.

The existence of the large supply itself suggests that something significant has changed with the kind of drug Missouri is using.

Compounded drugs can only last for a short period of time. The United States Pharmacopeia strongly recommends against keeping a compounded drug like pentobarbital for longer than 45 days — and that the drug can only last that long if it's frozen. Manufactured drugs, which are made by pharmaceutical companies, on the other hand, can last for months or years.

State officials changed drug suppliers in February 2014, after their previous supplier, the Apothecary Shoppe, was sued for, among other things, selling execution drugs when it wasn't licensed to do so in Missouri.

Until February 2014, Missouri's drug stockpile hovered around zero, presumably because the compounded drugs expired so quickly. Since changing drug suppliers, however, the state's drug supply has exploded, according to records obtained by BuzzFeed News.