The execution began at 7:53 p.m. (0153 GMT Friday) and Smith was declared dead at 8:25 p.m. (0225 GMT), prison officials said.
Smith appeared to remain conscious for several minutes after the nitrogen was activated, according to five journalists who were allowed to watch the execution through glass as media witnesses. Although the mask was also secured to the gurney, he then began shaking his head and writhing for about two minutes, and then could be seen breathing deeply for several minutes before his breathing slowed and became imperceptible, the witnesses said.
"It appeared that Smith was holding his breath as long as he could," Alabama Corrections Commissioner John Hamm told a press conference. "He struggled against the restraints a little bit but it's an involuntary movement and some agonal breathing. So that was all expected."
Rev. Jeff Hood, Smith's spiritual adviser, was at Smith's side for the execution, and said prison officials in the room "were visibly surprised at how bad this thing went."
"What we saw was minutes of someone struggling for their life," Hood, attending his fifth execution in the last 15 months, told reporters. "We saw minutes of someone heaving back and forth. We saw spit. We saw all sorts of stuff from his mouth develop on the mask. We saw this mask tied to the gurney, and him ripping his head forward over and over and over again."
Before the nitrogen was switched on, Smith made a lengthy final statement that began: "Tonight, Alabama caused humanity to take a step backward."