WASHINGTON — (CNN) Another Tesla Model X crashed while in Autopilot mode over the weekend, this time in Montana, the third serious accident apparently tied to the self-driving feature.

The crashes are calling the safety of such automatic driving features into question, just as they’re being incorporated into more and more cars on the road.

At least two federal safety agencies are looking into the most serious Tesla Autopilot crash, a fatal accident in Florida that took place in May.

The driver in Montana was headed from Seattle to Yellowstone National Park when he crashed on a two-lane highway near Cardwell, at 12:30 a.m. Saturday, said Montana State Trooper Jade Shope.

Neither the driver nor his passenger were injured in the accident, but it was serious enough that the car lost its front passenger side wheel.

“It’s a winding road going through a canyon, with no shoulder,” Shope told CNNMoney.

The driver told Shope the car was in Autopilot mode, traveling between 55 and 60 mph when it veered to the right and hit a series of wooden stakes on the side of the road.

Tesla confirmed that the data it has from the car shows it was in Autopilot mode, and that the driver likely did not have his hands on the wheel.

“No force was detected on the steering wheel for over two minutes after autosteer was engaged,” said Tesla, which added that it can detect even a very small amount of force, such as one hand resting on the wheel.

“As road conditions became increasingly uncertain, the vehicle again alerted the driver to put his hands on the wheel,” said Tesla.

“He did not do so and shortly thereafter the vehicle collided with a post on the edge of the roadway,” Tesla said Autopilot is best used on highways with a center divider or while in slow-moving traffic.

“We specifically advise against its use at high speeds on undivided roads,” it said.

Tesla states clearly in its owner’s manual that drivers should stay alert and keep their hands on the wheel to avoid accidents when the Autopilot feature is engaged.

The driver received a traffic citation for careless driving following the accident.

“He still needs to maintain control of the vehicle, even if it’s on Autopilot,” said Shope, who could not release the driver’s name.

On May 7, Joshua Brown was killed when his Model S hit the trailer of a semi-truck that was crossing the highway in front of him.

Tesla confirms that car was in autopilot mode at the time of the accident, and that neither Brown nor autopilot applied the brakes before the crash.

Both the National Highway Traffic Safety Administration and the National Transportation Safety Board are investigating that crash.

The NTSB, best known for examining airline crashes and train wrecks, has a team of investigators headed to Florida to look into the crash starting Tuesday, said spokeman Christopher O’Neal.

It investigates only about 25 to 30 highway crashes year.

“The interest in this accident relates to the use of automation,” he said. “There’s an opportunity to learn from the information about how automation is introduced into American highways.”

On Friday July 1, Detroit area art gallery owner Albert Scaglione, 77, flipped his Model X onto its roof while driving on the Pennsylvania Turnpike about 100 miles east of Pittsburgh.

He told CNNMoney that he was in autopilot mode, though he wouldn’t comment further on the accident.

Both Scaglione and his passenger, his son-in-law, were treated and released at a nearby hospital following the accident.

— CNNMoney’s Matt McFarland contributed to this report