The CBC hired National Technical Systems, based in Calabasas, Calif., to test-fire 41 X26 Tasers from seven U.S. police departments. Each gun was fired six times, and "four delivered significantly more current than Taser International says is possible," the broadcaster reported. All of the guns were manufactured before 2005, the CBC said.

Biomedical engineer Pierre Savard at the University of Montreal, designed the testing procedure based on Taser International's specifications and said he found the results worrying.

"When you combine an increased current intensity with a dart that falls right over the heart for somebody who has cardiovascular disease or other conditions such as using drugs, for example, it can all add up to a fatal issue," Savard said.

In a statement, Taser said it "intends to contact NTS to suggest that the tests be repeated to verify the results."