A Texas death row inmate linked to five killings is asking the U.S. Supreme Court to halt his sentence a day before his scheduled execution.

Garcia White faces lethal injection Wednesday for fatally stabbing twin 16-year-old girls at a Houston apartment where their mother also was killed.

The 1989 deaths went unsolved until White confessed following his arrest in 1995 for the killing of a Houston grocery store owner. Prosecutors also linked him to a prostitute's death.

In their appeal, Garcia's lawyers say the lethal injection procedure may be unconstitutional because Texas refuses to say where it gets its execution drug. They also question whether White was mentally competent to waive his right to a lawyer during interrogations.

Another Texas inmate is scheduled for execution Thursday for a separate crime.