A new study shows that some Taser stun guns can deliver a much bigger jolt of electricity than the manufacturer says is possible and could increase the risk of cardiac arrest by as much as half in some people.

The study done by researchers commissioned by the Canadian Broadcasting Corp. also concluded that even stun guns firing at expected electrical levels carry some risk of inducing cardiac arrest in some people.

The study was done by a Montreal biomedical engineer and doctors working for a U.S. defense contractor and examined 44 Taser stun guns obtained from seven undisclosed U.S. police agencies.

Taser International Inc., based in Scottsdale, called the study flawed.

"Regardless of whether or not the anomaly (high-firing guns) is accurate, it has no bearing on safety," Taser Vice President Steve Tuttle said.

The researchers' analysis contradicts Taser's position that electric shocks from the weapons cannot kill. The results also raise questions about quality control in the stun gun's manufacturing and decline in performance over time.

The study tests found that of the 44 Tasers tested, four would not fire at all or fired improperly and four others produced from 47 percent more to 58 percent more power than the manufacturer specified.

All four of those that fired beyond their expected capacity were sold in 2004, making them among the oldest tested. That raised questions about how the guns age and how they were made in the first place, said Pierre Savard, a biomedical engineer in Montreal who co-authored the report with two Chicago doctors.

Taser officials acknowledged the possibility of a higher-than-normal initial charge in weapons not first given a "spark test" to ensure they are in proper working condition. They insist this does not affect safety and cautioned Canadian Broadcasting in a memo not to use "engineering minutiae to confuse the (public) and create a false sense of controversy."

Magne Nerheim, another Taser vice president, challenged the study on significant areas. He said researchers failed to spark-test the gun before testing the power, which created exaggerated results "not representative of actual output."

He also said the guns were tested using an incorrect resistance level that does not reflect the effect of Taser shocks on a human.

In an interview with The Arizona Republic, Savard said previous studies may have understated the risk Tasers pose because there is little available research on the effects of the weapon on humans, especially those who have heart disease.

Although Savard said he recognizes the value of less-lethal weapons, he added that he is convinced Tasers can kill in some circumstances.

"Scientists who had evaluated the Taser to start with said, 'Well, there's zero probability of death.' I'm sure that's not the case," Savard told The Republic.

The doctors and engineers hired by the CBC to interpret the results determined the higher electrical current was enough to raise the risk of an irregular heartbeat to as much as 50 percent for those with existing heart troubles.

"I'm 100 percent certain that cardiac diseases increase the risk of death after receiving Taser shock. I think there's enough scientific evidence for that."

It's still unknown if illegal drugs also change the risk factor, Savard added.

The guns are used by more than 12,000 police agencies across the country, including many in Arizona. Many authorities credit the weapon with preventing deaths and injuries to officers and suspects.

But there have been more than 380 deaths following police Taser strikes in the United States and 26 in Canada since 2001. Medical examiners have ruled that a Taser was a cause, contributing factor or could not be ruled out in more than 30 of those deaths.

Taser has challenged those findings and maintains the stun gun is safe.

The findings that the stun guns can produce higher-than-design shock levels contradict repeated Taser statements.

Taser executives have said for years that the guns could not produce shocks greater than the manufacturer's specifications and that the stun gun would melt before producing high-level shocks.

"The device is calibrated such that it cannot output any more power. It's running at 100 percent," Taser Chief Executive Officer Tom Smith testified in May during a British Columbia government inquiry into Taser safety.

The inquiry was spurred by the 2007 death of a Polish citizen at Vancouver International Airport. The man stopped breathing within moments of being shocked twice by police.