WASHINGTON — U.S. investigators are debating whether to blame a Boeing jetliner’s design for helping cause a cascade of pilot mistakes in last year’s Asiana Airlines crash that killed three Chinese teenagers.

The sticking point within the National Transportation Safety Board (NTSB), in the days before its final decision is due, is over the extent to which Boeing’s automatic throttle contributed to the 777’s loss of speed before the jet slammed into a seawall in San Francisco on July 6, said three people with knowledge of the discussions. They asked not to be identified because they weren’t authorized to speak.

The safety board Tuesday will decide on the probable cause and other factors leading to Seoul-based Asiana’s crash, the first in the United States with passenger deaths in more than four years. While there’s little doubt the pilots made mistakes, a finding that equipment on the 777-200 jet confused them could open Boeing to greater liability in lawsuits and shade public opinion.

“There’s quite a flurry of activity” in the days leading up to the NTSB’s findings in a major accident,” said Peter Goelz, who served as managing director of the agency during the 1990s.

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Such debate is consistent with prior investigations as the NTSB’s five-member board and staff hammer out nuances of what caused tragedies, Goelz said.

The NTSB has several options to address the role of the auto-throttle design. It can include it as part of the cause, list it as a “contributing factor” or make recommendations for safety improvements that mention the equipment’s role.

The plane crashed after the Asiana pilots allowed it to get almost 40 mph slower than the target speed as they neared the runway.

Both Boeing and Asiana, which gave their recommended conclusions to the NTSB in March, laid primary responsibility for the accident on the pilots.

The airline and plane-maker differ over whether other issues were at play.

Boeing maintains the accident was solely the crew’s fault and would have been avoided if the pilots had aborted the landing, as required under airline procedures, Miles Kotay, a spokesman, said in a June 20 email.

“The airplane and all airplane systems were functioning as expected prior to impact and did not contribute to the accident,” Boeing said in its submission to the NTSB earlier this year.

Asiana urged the safety board in its March submission to find that the auto-throttle was a cause of the pilots’ mistakes. It also said the NTSB should recommend that the Federal Aviation Administration (FAA) require Boeing to add a cockpit warning when speeds get too slow.

Groups with a stake in the outcome have met with the safety board’s politically appointed members in recent weeks in an attempt to influence the process, as is often the case before conclusions are announced on major accidents, one person familiar with the proceedings said.

The Everett-made 777, like Boeing’s 737 and 787, has a suite of computer-driven tools in the cockpit to help reduce pilots’ workload. Such automation, which has been adopted in similar ways by Airbus, is credited with helping reduce accidents and improve efficiency.

At the same time, pilot misunderstandings about automation have been studied for 20 years and linked to accidents, said David Woods, a systems-engineering professor at Ohio State University.

“This is an old story,” Woods said.

Recent accidents, such as the Feb. 25, 2009, crash of a Turkish Airlines-operated Boeing 737-800 on approach to Amsterdam that killed nine, have been linked to confusion over automation, he said.

Woods called it the “creeping complexity” of the cockpit.

An FAA study released last year found that pilots’ reliance on cockpit-automation systems has led to occasional confusion and new safety risks.

In the San Francisco accident, Capt. Lee Kang Kuk, a veteran with Asiana who was being trained on the 777, accidentally switched off the jetliner’s automatic speed control as he was trying to descend, according to NTSB documents.

The plane’s tail broke loose as it hit the seawall, and the fuselage then bounced and spun down the runway. The three girls who died may have been flung from the jet because they weren’t wearing seat belts, according to statements to investigators by unidentified witnesses traveling with them.

More than 200 people were taken to hospitals. The plane held 291 passengers, 12 flight attendants and four pilots.

In almost all settings, the plane’s auto-throttle will prevent the jet from flying too slow, even if it’s switched off.

Lee, 45, didn’t realize he’d put the jet into a rare mode that disabled this safety measure by making a series of changes to the autopilot and throttles.

He “believed the auto-throttle should have come out of the idle position to prevent the airplane going below the minimum speed” for landing, the NTSB said in a summary of an interview with him. “That was the theory at least, as he understood it.”

Another Asiana captain, Lee Jung Min, 49, an instructor, was seated in the co-pilot’s seat.

The pilots neglected to notice a runway light system showing they were too low, didn’t monitor their speed and didn’t follow airline procedures to abort a landing if they weren’t properly lined up for touchdown, according to information released already by the NTSB.

The 777, the world’s largest twin-engine jetliner, entered commercial service in 1995. Last year’s accident was the first involving fatalities and only the third serious enough to destroy a 777.

A fourth 777 that is presumed destroyed is the still-missing Malaysian Air 777-200ER model, the same as the Asiana jet.

Boeing designed the auto-throttle to aid crews while not replacing them, John Cashman, chief pilot on the 777 program, told an NTSB board of inquiry Dec. 11. The crew is “ultimately responsible for the safe conduct of the flight,” he said.