Texas death row inmate Robert James Campbell's stay of execution was denied by a federal appeals court on Tuesday, paving the way for his scheduled execution later today.

The Fifth U.S. Circuit Court of Appeals wrote in their decision that "Campbell speculates that there are unknowns regarding the drug to be used," reports The New York Times. Unfortunately for him, they added that speculation "is not enough."

Campbell's lawyer filed for an appeal, hoping to drag the case before the country's highest court, while Texas Attorney General Greg Abbott followed in kind with the state's response.

Officials have previously said their drug was different from the ones in Oklahoma that led to last week's botched execution, where the inmate lived on for nearly another pain-filled hour before dying of a heart attack. Texas currently only uses a single lethal drug, instead of the three-drug concoction used elsewhere. The state switched in 2012 after stock of one of three drugs expired.

As previously reported, Campbell also was seeking the Texas Board of Pardons and Paroles to grant him clemency as he claims his IQ is low enough that he shouldn't be eligible for capital punishment.

Campbell was convicted in 1991 in the robbery, rape and murder of Alexandra Rendon. He was out on parole at the time and had a partner, who pleaded guilty in return for a 35-year prison sentence.