SAN FRANCISCO (Reuters) - A Tesla Model S car was in "Full Self-Driving" mode when it hit and killed a 28-year-old motorcyclist in the Seattle area in April, police said, making it at least the second accident involving the technology on which Tesla CEO Elon Musk is pinning his hopes.

The 56-year-old driver was arrested on suspicion of vehicular homicide based on his admission that he was looking at his cell phone while using the driver assistant feature, the police said in a statement.

"The case is still under active investigation," said Chris Loftis of Washington State Patrol, adding that drivers are responsible for the safe operation of their vehicles.

Musk has this year shelved Tesla's all-new affordable cars and increased his bets on self-driving vehicles, saying he will be shocked if Tesla can't achieve full self-driving capability next year.

"The overwhelmingly important thing is achieving unsupervised full self-driving", he said in an interview with the Tesla Owners of Silicon Valley club last weekend.

He said a future vehicle will be like a "tiny mobile lounge" where drivers will be able to watch movies, play video games, work and even drink and sleep.

Musk has been aiming to achieve self-driving capability for several years, with the technology under growing regulatory and legal scrutiny.

Tesla calls its driver assistant features Autopilot or "supervised" Full Self-Driving, but says they require active driver supervision and do not make its vehicles autonomous.

The National Highway Traffic Safety Administration (NHTSA) previously said there was one fatal accident between August 2022 and August 2023.

The regulator began a probe of Autopilot in August 2021 after identifying more than a dozen crashes in which Tesla vehicles had hit stationary emergency vehicles, and reviewed hundreds of crashes involving Autopilot.

Tesla was forced to recall nearly all its vehicles on U.S. roads over a lack of safeguards.