Arizona Attorney General Tom Horne's office said Wood, 55, was pronounced dead at 3:49 p.m., one hour and 57 minutes after the execution started.
It is the third prolonged execution this year in the U.S., including one in Ohio in which an inmate gasped in similar fashion for nearly a half-hour. An Oklahoma inmate died of a heart attack in April, minutes after prison officials halted his execution because the drugs weren't being administered properly.
Gov. Jan Brewer said later that she was ordering a full review of the state's execution process, saying she's concerned by how long it took for the administered drug protocol to kill Wood.
An Associated Press reporter who witnessed the execution saw Wood start gasping shortly after a sedative and a pain killer were injected into his veins. He gasped more than 600 times over the next hour and a half. During the gasps, his jaw dropped and his chest expanded and contracted.
An administrator checked on Wood a half dozen times. His breathing slowed as a deacon said a prayer while holding a rosary. Wood finally stopped breathing and was pronounced dead 12 minutes later.
"Throughout this execution, I conferred and collaborated with our IV team members and was assured unequivocally that the inmate was comatose and never in pain or distress," said state Department of Corrections Director Charles Ryan.
Defense lawyer Dale Baich called it a botched execution that should have taken 10 minutes.
"Arizona appears to have joined several other states who have been responsible for an entirely preventable horror — a bungled execution," Baich said. "The public should hold its officials responsible and demand to make this process more transparent."
Family members of Wood's victims in a double 1989 murder said they had no problems with the way the execution was carried out.
"This man conducted a horrific murder and you guys are going, let's worry about the drugs," said Richard Brown, the brother-in-law of Debbie Dietz. "Why didn't they give him a bullet, why didn't we give him Drano?"
Wood looked at the family members as he delivered his final words, saying he was thankful for Jesus Christ as his savior. At one point, he smiled at them, which angered the family.
Arizona uses the same drugs — the sedative midazolam and painkiller hydromorphone — that were used in the Ohio execution earlier this year. A different drug combination was used in the Oklahoma case.
"These procedures are unreliable and the consequences are horrific," said Megan McCracken, of the University of California, Berkeley, School of Law's Death Penalty Clinic.
States have refused to reveal details such as which pharmacies are supplying lethal injection drugs and who is administering them out of concerns that the drugmakers could be harassed.
Wood filed several appeals that were denied by the U.S. Supreme Court. Wood argued he and the public have a right to know details about the state's method for lethal injections, the qualifications of the executioner and who makes the drugs. Such demands for greater transparency have become a new legal tactic in death penalty cases.