The American Civil Liberties Union is suing Missouri over a statute that prohibits the identification of those involved in prisoner executions without the permission of the Corrections Department director.

The lawsuit, filed Wednesday by the St. Louis-based ACLU-Missouri, says the statute is unconstitutional.

The proposed use of propofol drew concerns from the medical community because most of the drug is made in Europe, and the anti-death penalty European Union has threatened to limit export if propofol is used in an execution.

The new protocol makes a compounding pharmacy part of the execution team, and the statute protects the anonymity of the pharmacy.

“The government’s trend toward unwarranted secrecy that conceals practices of doubtful constitutionality needs to stop,” Jeffrey A. Mittman, executive director of the ACLU-Missouri, said in a statement.

Messages left Thursday with officials with the Corrections Department and the Missouri Attorney General’s office were not returned.

Nixon’s decision stopped the execution of convicted killer Allen Nicklasson scheduled for Oct. 23. The execution of another convicted killer, Joseph Franklin, is scheduled for Nov. 20, and would be the first in Missouri using a single drug.

Missouri has previously used a three-drug cocktail of drugs for executions in the modern era. Those drugs have become increasingly difficult for prisons and corrections departments to obtain because the makers are opposed to their use in executions. As a result, Missouri and most other states have switched to single-drug executions.

The Missouri Department of Corrections earlier this month turned over hundreds of pages of documents related to the use of propofol to the ACLU in response to another lawsuit. The documents were posted on the ACLU website and included the names of companies that provided propofol and other drugs to the state.

ACLU later removed the documents from its website, citing fear of violating the state statute.