SALT LAKE CITY — A Hurricane police officer won't face prosecution for using a Taser on a Northern Arizona University graduate student who later died, the Washington County attorney's office said Thursday.

Officer Ken Thompson acted in accordance with his training, County Attorney Brock Belnap said in a letter to Hurricane Police Chief Lynn Excell.

Brian Cardall, 32, of Flagstaff, who suffered from bipolar disorder, died June 9 after he was hit twice with a Taser on state Route 59 in southern Utah. Cardall was running naked in traffic on the highway.

A Taser, a type of stun gun, shoots a probe that delivers an electric shock.

"In my view, officer Thompson responded to a tense, uncertain and rapidly unfolding situation in a manner consistent with his training and that he did not intend, believe or foresee would seriously injure or cause the death of Brian Cardall," Belnap wrote in the letter.

The decision comes after an investigation by the county's critical incident task force and a review by a panel of prosecutors. In his letter, Belnap said the unanimous recommendation of the panel was not to file charges.

An autopsy report said Cardall died from heart irregularities and that his "manic episode" had put him at risk of "sudden cardiac death." Assistant Utah Medical Examiner Erik Christensen only said the Taser's deployment "could" have been a primary or underlying factor in Cardall's death.

"The circumstances in this case represent a combination of factors that are believed to increase the risk of potential electrical death," Christensen said in a portion of the autopsy released through Belnap's office.

According to the investigative summary, barbs from the Taser stuck Cardall in the left chest, over his heart.

Tasers are manufactured by Taser International in Scottsdale. Last month, the company began advising police agencies to avoid firing the device at a person's chest, saying that doing so will make defending lawsuits easier. The manual includes a lengthy explanation about deaths caused by sudden cardiac arrest.

"Should sudden cardiac arrest occur in a scenario involving a Taser discharge to the chest area, it would place the law enforcement agency, the officer, and Taser International in the difficult situation of trying to ascertain what role, if any, (the device) could have played," according to the manual.

Tasers are used by 14,200 law enforcement agencies throughout the United States. Human rights groups have long said that Tasers cause heart attacks.

OTHER LEGAL OPTIONS

In a statement issued through an attorney, Cardall's family said it was disappointed, but not surprised, by Belnap's decision.

They criticized the county's report for glossing over or ignoring some facts and eyewitness accounts of Thompson's 42-second interaction with Cardall before the Taser was deployed. According to the family, some eyewitness account contradict statements made by police about Cardall's behavior.

"In light of the county attorney's decision, the family will discuss their options for ensuring that the full truth emerges regarding Brian's death," the statement from attorney Karra Porter reads. "They also want to feel reassured that steps are being taken to prevent other senseless deaths in the future."

In police recordings, Cardall is heard screaming as his wife, Anna Cardall, asks a 911 dispatcher for help before police arrived. She also tells the dispatcher her husband had been diagnosed as bipolar and that she had given him some medicine.

The tapes also recorded Thompson yelling at Cardall to "get down" at least a dozen times before the Taser is deployed. Later, Thompson is heard saying he deployed the device a second time when a downed Cardall tried to get up.

At a news conference late Thursday, Excell expressed his sympathy for the Cardall family, but said the report vindicates the officer's actions. An attorney for the city declined to answer questions, citing a concern about future litigation.

"We view this as a very deep human tragedy for everyone involved," attorney Peter Stirba said. "Certainly the officers have been traumatized by the events and certainly the Cardall family has sustained a profound loss."

FEW KNEW OF DISEASE

Cardall was a graduate student at NAU in molecular ecolog. He'd also recorded music CDs under the pseudonym of "Jonahs," had a flair for writing and oil painting, was a veteran rock climber and never missed a chance to introduce his toddler to his love of science.

Faculty members at Northern Arizona University, where Cardall was studying how genetic mechanisms drive biological diversity in plants and animals, reacted in June with shock to his death.

Cardall was pursuing his doctoral degree at the university by earning a Science Foundation Arizona Fellowship — a prestigious scholarship given to Cardall, who beat out several other standout candidates to achieve the award.

Brian's father, Duane Cardall, said his son was driving home from a family wedding in Salt Lake City with his wife Anna when Brian, recently diagnosed with bipolar disease, became disoriented.

The Cardalls' vehicle pulled over to the side of the road so Brian could take some medication. His confusion continued, however, and he left the car, Duane Cardall said.

Duane Cardell said Anna Cardall, who was six months pregnant with the couple's second child at the time, was never threatened or in danger because of her husband's behavior.

He said few people knew of his son's recent illness, which was diagnosed after a breakdown brought on by "a time of extraordinary academic stress" while he completed a master's degree at Utah State University, Cardall said. Brian Cardall rarely exhibited any symptoms of his illness, his father said.

Duane Cardall said his son spent his final days toting his daughter, Ava, with him as he did field work along the Virgin River near St. George. The two collected bugs together.

"He was an amazing teacher and even though Ava was only 2, he was conveying to her information to help her develop those inquisitive tendencies," Cardall said.

File material from the Salt Lake City Tribune was included in this AP report