An elite hacker who was due to demonstrate how heart implants could be hacked has died unexpectedly in San Francisco.
Barnaby Jack died on Thursday, the city's medical examiner's office told Reuters, but did not give more details.
He had been due to give a presentation into medical device vulnerabilities at the Black Hat security conference in Las Vegas taking place next week.
He had said one technique could kill a man from 30 feet (nine metres) away.
IOActive, the security firm at which Mr Jack was director of embedded devices, said it was preparing a statement.
In a tweet, the company said: "Lost but never forgotten our beloved pirate, Barnaby Jack has passed."
His sister Amberleigh Jack, who lives in New Zealand, told Reuters news agency he was 35.
Mr Jack became one of the most famous hackers on the planet after a 2010 demonstration in which he hacked a cash machine, making it give out money. The technique was dubbed "Jackpotting".
'Social media flood'
More recently, he emerged as a leading expert in the weaknesses that could be found in medical technology.
Last year, he told the BBC about how he had discovered flaws in widely-used insulin pumps which allowed him to compromise the devices.
The hack made it possible to control them and administer a fatal level of insulin, Mr Jack said.
"My purpose was not to allow anyone to be harmed by this because it is not easy to reproduce," he told the BBC during an interview in April 2012.
"But hopefully it will promote some change in these companies and get some meaningful security in these devices."
Mr Jack's expertise and vivid demonstrations of his knowledge at events like Black Hat earned him the respect of many security professionals.
Amberleigh Jack thanked those who have been posting messages of sympathy online.
"So humbled by the social media flood of people that loved @barnaby_jack," she tweeted.
"Thank you all so much for your kind words."