Sells, who claims to have killed dozens of people, was scheduled to be put to death Thursday for stabbing Kaylene "Katy" Harris, 13, in Del Rio, Texas, on Dec. 31, 1999. Hernandez-Llanas, 44, was convicted of the 1997 killing of a Kerrville rancher who had allowed him to live on his property.
Texas officials said they are likely to appeal the ruling.
Death-penalty states have been scrambling to find execution drugs because many pharmaceutical companies now refuse to supply them. Texas planned to use pentobarbitol, a sedative, to kill Sells and Hernandez-Llanas.
Lawsuits have been filed in a number of states by inmates denied information on the suppliers of death penalty drugs. State officials say they need to protect the compounding pharmacies they use from threats and potential violence.