Trooper Jon Gardner remains on leave, primarily for his safety, after numerous anonymous threats were made against him, said Supt. Lance Davenport of the Utah Highway Patrol.
Gardner twice zapped Jared Massey with a Taser when the driver walked away and refused to sign a speeding ticket on Sept. 14. The incident was recorded on Gardner's dashboard camera. Massey filed a public-records request and posted the video on YouTube, which said it has been viewed more than 1 million times.
"We found that Trooper Gardner's actions were lawful and reasonable under the circumstances," Davenport said at a news conference, joined by Scott Duncan, commissioner of the UHP's parent agency, the Utah Department of Public Safety.
The investigation was conducted by officials in the Department of Public Safety, which oversees the highway patrol. The officials have asked the Utah attorney general's office to also review the case to determine if laws were broken.
Massey was not at the news conference and could not immediately be reached for comment.
The video showed Massey arguing about whether he was exceeding the speed limit on U.S. 40 in eastern Utah. Massey got out and walked to the rear of his vehicle. The trooper pulled out his Taser when the driver tried to return to his seat.
Massey shrieked, fell and said: "Officer, I really don't know what you're doing."
"Face down! Face down! Put your hands behind your back," Gardner said.
When Massey's wife emerged from the passenger side, the trooper ordered her to get back in - "or you're going to jail, too." Moments later, when another officer arrived, one of them said, "Oh, he took a ride with the Taser."
Davenport said that comment was inappropriate.
Officials said Gardner could have issued the ticket without Massey's signature. The investigation found use of the Taser was justified because Massey had turned his back and put a hand near his pocket, Davenport said.
"For a law-enforcement officer, that is a very, very scary situation," he said.
Nonetheless, the trooper now realizes that other options were available, Davenport said.
The UHP has received thousands of phone calls and e-mails since the video was posted online, many of them critical of the trooper. There also have been online threats against Gardner.
"I think mostly it's people blowing off steam, and that's fine," Sgt. Jeff Nigbur, a UHP spokesman, said Friday before the investigation's conclusions were announced. "But you can't say you're going to endanger somebody's life."
He said there was no evidence that the trooper's life is in danger. Massey has pleaded for the online threats to stop.
Tasers use compressed nitrogen to fire two barbed darts that can penetrate clothing to deliver a 50,000-volt shock to immobilize people.
Tasers are manufactured by Taser International of Scottsdale.