A media advocacy group and the American Civil Liberties Union are asking Missouri's highest court to settle whether the state's prison system must reveal the closely guarded source of the drug it uses in executions.

The nonprofit Reporters Committee for Freedom of the Press, the American Civil Liberties Union and other plaintiffs asked the Missouri Supreme Court on Wednesday to review the matter, saying the issue has led to conflicting lower-court rulings.

The Missouri Department of Corrections has refused to disclose who supplies it with pentobarbital, saying suppliers' identities are shielded as part of its "execution team."

But the sources of the drugs in Missouri and other death-penalty states are widely believed to be compounding pharmacies, which make drugs tailored to a client's specific needs. Those pharmacies do not face the same approval process or testing standards of larger pharmaceutical companies, spawning lawsuits by watchdogs pressing for them to be publicly known and properly scrutinized.

Wednesday's filing insisted that "any resolution of this question directly affects the ability of the public to exercise effective oversight."

"Transparency is critical for the public to maintain trust in the manner in which executions are carried out in this state," the filing added. "Given this court's special, constitutionally enshrined role in monitoring executions in Missouri, it is in the best position to resolve this issue of immense public interest."

The Missouri Department of Corrections has not returned calls left Thursday morning seeking comment.

The Missouri Court of Appeals' Western District ruled in February that prison administrators aren't obligated to divulge who supplies the execution drugs, overturning a 2016 trial court ruling that found the state wrongly withheld documents that would identify pharmaceutical suppliers.