A Tesla Model S car was in ‘Full Self-Driving’ mode (FSD) when it was involved in a fatal crash in Seattle, Washington in April this year, Washington State Police have confirmed.

The driver, who was arrested on suspicion of vehicular homicide, admitted he was looking at his mobile phone while using the driver assistant feature.

Tesla says its "Full Self-Driving (Supervised)" software requires “active driver supervision” and does not make vehicles autonomous, Reuters reports.

Currently vehicles with self-driving capabilities are not allowed in Washington, a Washington State Patrol spokesman told USA TODAY.

Only three companies are certified for testing-only in the state, NVIDIA, Waymo and Zoox, which have been authorised to do so by the Department of Licensing.

According to the US National Highway Traffic Safety Administration (NHTSA), there had previously been one fatal accident involving a Tesla vehicle using FSD software between August 2022 and August 2023.

The NHTSA said that it "is aware of this crash and is gathering information from local law enforcement and Tesla."

Washington State Police said the case remains under investigation.