The incident occurred on Wednesday in the town of Ratzeburg, German police said.

The car’s driver told Tesla (TSLA) following the crash that he was using Autopilot when the incident occurred but that it was unrelated to the accident, a Tesla spokeswoman said on Thursday.

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Tesla’s Autopilot, introduced last October, helps drivers steer and stay in lanes and is intended to assist, but not replace, drivers. It has been the focus of intense scrutiny since it was revealed in July that a Tesla Model S driver was killed while using the technology in a May 7 collision with a truck in Florida.

Since then, there have been various reports of accidents involving Teslas whose drivers were using Autopilot, including a fatality in China, but it is not clear what role, if any, the driving assistance system played. There have also been reports of serious accidents prevented because of Autopilot.

Police in Ratzeburg said in a statement on Thursday that the 50-year-old driver from Brandenburg drove into the back of a Danish tour bus as it was returning to the inside lane after overtaking. Police said they planned to investigate “why (Autopilot) did not work.”

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The Tesla driver was slightly injured but none of the 29 bus passengers were hurt, the statement added. Authorities were not immediately reachable by telephone after hours on Thursday.

Tesla denied that Autopilot was at fault, saying the bus swerved into the car’s lane and side-swiped the Tesla, making a collision “unavoidable,” the spokeswoman said.

“We can only do so much to prevent an accident,” she said, adding that Tesla was in contact with German police.

The Silicon Valley automaker said this month it was updating the semi-autonomous driving system with new limits on hands-off driving and other improvements.

This story has been updated as details have developed.