The vast majority of the country's 32 death penalty states refuse to disclose the source of their execution drugs.
A review by The Associated Press has found that the states cloaked in secrecy include some with the most active death chambers. Texas, Florida, Oklahoma and Missouri are among them.
The secrecy comes as most states now rely on loosely regulated "compounding pharmacies" for execution drugs but refuse to name them. They cite concerns about backlash that could endanger the supplier's safety.
Defense attorneys question how an inmate's constitutional right against cruel and unusual punishment can be guaranteed if nothing is known about the drug being used to kill him.
Proponents say forcing states to reveal their drug source can amount to obstruction of justice by delaying executions.