Charles Thompson, 35, slipped out of the purportedly high-security unit of the Harris County jail in Houston on Thursday - less than a week after he was sentenced to death following a retrial for two murders committed in 1998.
Officials are investigating whether he received help from a prison employee to obtain the clothes and an identification card which suggested he worked for the Texas Attorney General's office. He was last seen leaving the facility wearing a blue shirt, khaki trousers and white tennis shoes.
"He still has not been found," Lt John Martin of the Harris County sheriff's department, told the Houston Chronicle. "He managed to get some civilian clothing. He had changed out of the orange jumpsuit that inmates ordinarily wear. We don't know exactly where he got that from."
He added: "It definitely suggests this was pre-planned and that he had help in making his escape."
Officials said Thompson's jumpsuit was found in a room used by lawyers to meet with their clients. It appears he used the room to change his clothes. It is not clear how he obtained access to the room.
Thompson was sentenced to death last Friday for the murders of Dennise Hayslip and Darren Cain. He was due to be moved within weeks to the Polunsky Unit in Livingston, which houses Texas's male death-row inmates.
Ms Hayslip's brother, Mike Donaghy, said: "This is something that's just unfathomable to me. How can a convicted murderer get away like that? How did he get in civilian clothes? How did he get a badge?"