The security researcher who hacked into a moving Jeep earlier this year has resigned as an engineer at Twitter after three years on the job, a media report said.

Charlie Miller, a former National Security Agency hacker, reportesdly one of the world's best known security experts, declined to comment on his departure or say what he would do next, Reuters said.

Miller's latest feat, breaking into a moving Jeep as it drove on the highway, was done with IOActive researcher Chris Valasek and was the subject of talks at this month's security conferences in Las Vegas.

Their efforts, which were coordinated with Fiat Chrysler Automobiles, prompted the first vehicle recall to protect drivers from possible malicious hacking.

FCA and supplier Harman are now under regulatory scrutiny after the automaker reportedly delayed 18 months before telling NHTSA about the security flaw in radios fitted in over 1m vehicles.

FCA is updating software in approximately 1.4m US vehicles equipped with certain types of radio after it was shown hackers could gain control of the car.