MIAMI (REUTERS) - When police spotted Mr Israel Hernandez Llach spray-painting a shut-down McDonald's in August, the Miami teenager decided to make a run for it. Moments later, the unarmed, 18-year-old graffiti artist was dead. He had been struck in the chest by a police stun gun.

The Florida Department of Law Enforcement is still investigating what caused the Aug 6 death of the Colombian-born teenager. His death in Miami Beach, where police conduct has come under intense scrutiny in recent years, has triggered protests calling for a change in the way officers use the stun guns known as Tasers. It has also reignited a debate about whether the electrical shock the Taser delivers can sometimes trigger a cardiac arrest when fired at the chest area.

"The fact that he was shot in the chest is something we are analysing," said Mr Jose Rodriguez, a lawyer for Mr Hernandez Llach's family. "We're working with the assumption for now that the Taser caused his death."

Tasers, used by police officers in the United States and globally, have been the target of criticism from advocacy groups like Amnesty International and the American Civil Liberties Union, which argue they can be lethal and have called for more stringent rules on their use. Proponents of Tasers say stun guns are a very useful tool for law enforcement officers, enabling them to subdue suspects without deadly force.

"It's not a magic bullet," said Mr Steve Tuttle, a spokesman for Taser International. "But it is the most effective, accountable tool that officers have."

A study published last year in the American Heart Association journal, Circulation, analysed the cases of eight people in the US who suffered cardiac arrest after being shocked by a Taser in the chest. Seven of the people died and one survived, leading the study's author to conclude that electricity delivered by a Taser can speed up the heart rate and provoke cardiac arrest in some cases.

"Cardiac arrest can happen," said Dr Douglas Zipes, a cardiologist and distinguished professor at the Indiana University School of Medicine who led the study. "It's infrequent, but how infrequent, we don't know."

Taser International has questioned the study's results, saying it fails to establish a clear connection. In 2009, the company adjusted the guidance it provides to police departments on Taser use, warning officers to avoid, if possible, shots to the chest because of extremely low risk of an "adverse cardiac event".

"If there is going to be a cardiac arrest, it's going to be extraordinarily rare," Mr Tuttle said.

In its safety materials, Taser recommends users aim away from the chest area, as well as the head, throat and any areas with an existing injury, if possible.

Mr John Burton, a California lawyer who has successfully tried several cases against Taser involving chest shots, said the language needs to be clearer about potential risks, and more data is needed.

"No one is going around collecting these incidents, studying their frequency, studying what happened," he said.

When a person dies from being Tasered, sometimes the cause of death is not clear, Mr Burton said.

Mr Hernandez Llach's father, Mr Israel Hernandez Bandera, is awaiting a medical examiner's report that could reveal the cause of death. The report's findings are being withheld because of a civil suit the family filed accusing the police of excessive force. Such withholding is normal when suits are filed.

"I just want some answers," he said. "Was it the Taser? Was it something else?"