The driver of a Tesla electric car had the vehicle's semi-autonomous Autopilot mode engaged when she slammed into the back of a Utah fire truck over the weekend, in the latest crash involving a car with self-driving features.

The 28-year-old driver of the car told police in suburban Salt Lake City that the system was switched on and that she had been looking at her phone before the Friday evening crash.

Tesla's Autopilot system uses radar, cameras with 360-degree visibility and sensors to detect nearby cars and objects. It's built so cars can automatically change lanes, steer, park and brake to help avoid collisions.

The auto company markets the system as the "future of driving" but warns drivers to remain alert while using Autopilot and not to rely on it to entirely avoid accidents. Police reiterated that warning Monday.

A Tesla spokesperson did not comment following the disclosure about the use of the feature.

On Twitter, co-founder Elon Musk said it was "super messed up" that the incident was garnering public attention, while thousands of accidents involving traditional automobiles "get almost no coverage."

Elon Musk tweet

South Jordan police said the Tesla Model S was going 60 mph (97 kph) when it slammed into the back of a fire truck stopped at a red light. The car appeared not to brake before impact, police said.

The driver, whom police have not named, was taken to a hospital with a broken foot. The driver of the fire truck suffered whiplash and not taken to a hospital.

"What's actually amazing about this accident is that a Model S hit a fire truck at 60 mph and the driver only broke an ankle," Musk tweeted. "An impact at that speed usually results in severe injury or death."

Elon Musk tweet

The National Transportation Safety Board has not opened an investigation into the crash, spokesman Keith Holloway said, though it could decide to do so.

Over the past two months, federal officials have opened investigations into at least two other crashes involving Tesla vehicles.

Last week, the NTSB opened a probe into an incident in which a Model S caught fire after crashing into a wall in Florida.

Two 18-year-olds were trapped in the vehicle and died in the flames. The agency has said it does not expect the semi-autonomous system to be a focus of that investigation.

The NTSB and the National Highway Traffic Safety Administration are also looking into the performance of the company's Autopilot system in the March crash of a Tesla Model X on a California highway. The driver in that incident died.

The investigation into the crash in Utah is ongoing, police said.

The driver of the Tesla may face charges for failing to maintain the safety of her vehicle, which would be a traffic infraction, according to police spokesman Sgt. Samuel Winkler.