WASHINGTON — Asiana Flight 214’s pilots caused the crash last year of their airliner carrying more than 300 people by bungling a landing approach in San Francisco, including inadvertently deactivating the plane’s key control for airspeed, the National Transportation Safety Board (NTSB) concluded Tuesday.

But the board also said the complexity of the Boeing 777’s automated flight control system contributed to the accident. Materials provided to airlines by Boeing failed to make clear under what conditions the autothrottle doesn’t automatically maintain speed, the report said.

The findings bring to a close a U.S. investigation into the July 6, 2013 crash, which killed three passengers and triggered questions about South Korea’s pilot training and one of Boeing’s most popular jets. The crash was the first commercial-airline accident in the U.S. with passenger fatalities since 2009 and the first fatal accident involving a Boeing 777.

The board’s acting chairman, Chris Hart, warned that the accident underscores a problem that has long troubled aviation regulators worldwide — that increasingly complicated automated aircraft controls designed to improve safety are also creating new opportunities for error.

The Asiana flight crew “over-relied on automated systems that they did not fully understand,” Hart said. “As a result, they flew the aircraft too low and too slow and collided with the seawall at the end of the runway.” Asiana’s pilot training also was faulted.

Of the 307 people on board Flight 214, three Chinese teens were killed in the crash.

Nearly 200 were injured, including 49 seriously.

Asiana Airlines said that it has already implemented the NTSB’s training recommendations, and that it agreed with the finding that one factor was the complexity of the autothrottle and autopilot systems, as well as their descriptions in Boeing training manuals.

The jet-maker immediately rejected the notion that the 777’s automated systems contributed to the accident, pointing to the aircraft’s safety record.

“Boeing respectfully disagrees with the NTSB’s statement that the 777’s auto-flight system contributed to this accident,” the company said in a statement.

“The auto-flight system has been used successfully for over 200 million flight hours across several airplane models, and for more than 55 million safe landings. The evidence collected during this investigation demonstrates that all of the airplane’s systems performed as designed.”

The board, which made 27 recommendations to prevent future disasters, didn’t say the autothrottle failed to perform as designed but rather that its design, under certain circumstances, could lead to confusion as to whether it was controlling speed or in an inactive state.

Investigators said the flight’s three veteran pilots made 20 to 30 different errors, some minor and others significant, during the landing approach.

Among the errors were that pilots didn’t follow company procedures when they failed to call out notifications about the plane’s altitude, speed and actions they were taking during the landing approach.

They also weren’t closely monitoring the plane’s airspeed — a fundamental of flying. Instead, they assumed the autothrottle was maintaining the required speed for a safe landing.

But the captain flying the plane, Lee Kang Kuk, 45, who was new to the 777, inadvertently prevented the autothrottle from controlling the plane’s speed. He put the throttle in idle after the plane had unexpectedly climbed too high.

He assumed the throttle would automatically resume controlling speed, as it is designed to do under most circumstances. But because he turned off the autopilot at the same time, the autothrottle remained on hold at the last selected speed, which was idle.

A training captain who was sitting next to Lee in the right seat didn’t notice the error, then compounded it by turning off only one of two other key systems for managing the flight. Both systems are supposed to be on or off, but not one on and one off.

A third pilot riding in the jump seat noticed that the plane was descending too fast but didn’t speak up right away.

The pilots also didn’t immediately start to abort the landing when they realized something had gone awry. By the time they did call for a “go-around” it was too late.

The plane struck a seawall and its tail was ripped off. The rest of the plane went spinning and sliding down the runway.

Two of the three teens who were killed weren’t wearing their seat belts and were thrown from the plane.

The third teen was hit on the head by one of the plane’s doors but survived the crash.

She was killed when, lying unconscious, she was run over by two San Francisco Fire Department vehicles in the chaos afterward.

The NTSB made a series of recommendations, including that Boeing should develop enhanced training on the use of the autothrottle and also that it should consider modifying the automatic flight control systems to ensure that air speed doesn’t fall below a minimum.

Boeing reacted with some wariness to this suggestion.

“We are committed to a process of continual improvement of our airplanes, and we will carefully review the NTSB’s recommendations,” its statement said. “However, it is important that any recommendation concerning changes to the airplane’s design be reviewed with great care, and with due consideration for the potential unintended consequences of any change.”

Material from Bloomberg News is included in this report. Times aerospace reporter Dominic Gates also contributed.