The federal lawsuit says the only reason San Quentin State Prison officials inject a paralyzing agent is to sanitize the execution and prevent witnesses from perhaps seeing convulsions.
The paralyzing drug, according to the lawsuit, "makes it impossible for witnesses to determine whether death row inmates in California are being subjected to substantial and unnecessary pain before dying."
The induced paralysis, the group argued, conceals significant information to which the public is entitled.
The ACLU, which filed the suit on behalf of San Francisco-based Pacific News Service, made a similar argument a year ago before the 9th U.S. Circuit Court of Appeals on behalf of a condemned inmate. The court rejected it on procedural grounds, and did not reach the merits of the challenge.
In response to Wednesday's filing, Nathan Barankin, spokesman for Attorney General Bill Lockyer, said: "The ACLU does not have a right to determine what method the state of California should use in carrying out the death penalty." He said the "paralyzing agent effectively stops inmate breathing."
Under California's protocol, a sedative, then the paralyzing agent and finally heart-stopping drugs are injected. The state is seeking court permission to drip the sedative continually to ensure unconsciousness.
Wednesday's lawsuit comes two weeks after the execution of Michael Morales was called off amid questions of whether California inmates suffer too much pain during execution in violation of the Eighth Amendment ban on cruel and unusual punishment.
Litigation is ongoing in the Morales case, with hearings on California's injection method set to begin May 2 in San Jose federal court. The U.S. Supreme Court has never addressed whether lethal injection amounts to cruel and unusual punishment, and 37 states use an injection protocol similar to California's.