AURORA | The American Civil Liberties Union of Colorado filed a lawsuit Tuesday against the state prison system aiming to find out details about how the state plans to kill Nathan Dunlap.

The Department of Corrections has not said publicly what mix of drugs will be used to kill Dunlap, who gunned down four people at an Aurora Chuck E. Cheese restaurant in 1993.

Lawyers for the ACLU said the DOC’s refusal to disclose details about their execution plan violate the public’s right to know.

“By refusing to disclose the details of the execution procedure, including the drug or drugs that may be used and how they are obtained, as well as information about the companies that may be supplying the chemicals, CDOC infringes, without adequate justification, on the public’s legitimate right to information about how its government operates with regard to one of its most serious undertakings,” ACLU of Colorado Legal Director Mark Silverstein said in a statement announcing the lawsuit Tuesday.

Dunlap is scheduled to be executed in August. If the execution goes through, it will be the first in Colorado since 1997 and just the second in the state since the U.S. Supreme Court reinstituted the death penalty in the 1970s.

Lawyers for Dunlap have already challenged the state’s execution procedures, but those efforts have largely failed.

In April, the state court of appeals rejected the defense’s arguments that the DOC didn’t follow state rules in establishing their lethal injection protocols and that details about how an execution will be carried out remain secret. The details about carrying out a lethal injection are the DOC director’s responsibility, the courts said, and judges don’t have jurisdiction over those details.

The defense has appealed that ruling to the Colorado Supreme Court.

The ACLU lawsuit, filed in Denver District Court, comes after prison officials rejected an ACLU request for communications between the DOC and compounding pharmacies, as well as a document outlining the execution protocol and training given to prison staff.

The documents are important, the DOC argued, because they could lead to further discussion of the state’s death penalty and the “ethical implications of pharmacists preparing and providing execution drugs.”

Colorado law mandates that executions be carried out via lethal injection and according to the DOC, executioners use a lethal cocktail of sodium thiopental, pancuronium bromide and potassium chloride.

But experts say thiopental is no longer available, so states have to find a different mix of drugs for their executions. The DOC has not said publicly what mix of drugs they plan to use to execute Dunlap.

The ACLU said prison officials last month sent a letter to pharmacists requesting “sodium thiopental, pentobarbital, pancuronium bromide, potassium chloride, or similar drug or drugs that the CDOC can purchase.”

That request, the ACLU argued, could be a sign that the DOC plans to use a three-drug cocktail to kill Dunlap, even though state law calls for just one drug.

Three-drug cocktails have garnered criticism in recent years from death penalty opponents who say they cause extreme pain. A judge in 2006 halted executions in California because of those concerns and that state’s death penalty remains in legal limbo as courts decide its legality.

The ACLU is also raising concerns about what role pharmacists play in providing DOC with the drugs to kill Dunlap.

The complaint points to Colorado Board of Pharmacy Rules of Professional Conduct which prohibit “any practice which detrimentally affects the patient.” The rules also say a “pharmacist may not dispense a prescription drug or a controlled substance based on an order that does not list a specific patient,” the ACLU said.

“If compounding pharmacies in Colorado plan to participate in Colorado’s next execution, the public has a right to know,” said Silverstein, “and the State Board of Pharmacy might be interested in investigating whether pharmacies violate their license and state rules when they supply a drug for the express purpose of execution.

This isn’t the first time drug makers involved in lethal injection have come under fire. The Italian company that used to produce thiopental stopped producing the drug amid pressure from leaders who oppose the death penalty in the deeply Catholic country.