In a second day of congressional hearings into Boeing’s handling of its ill-fated 737 Max plane, lawmakers were shown internal records revealing that three years before two fatal crashes one employee had expressed concern that an anti-stall flight system at the center of crash investigations could be triggered by a single sensor.

“Are we vulnerable to single [angle-of-attack] failures with [the system’s] implementation or is there some checking that occurs?” a Boeing employee asked in an email from December 2015, nearly three years before the first Max 737 crash in Indonesia almost exactly a year ago.

The employee, who has not been identified, was referring to a sensor on the outside of the plane that measured its angle in flight and could trigger a system, known as MCAS, to push it down if it thought the aircraft was at risk of stalling.

Relying on a single sensor, aviation experts say, is fundamentally dangerous. With no back-up any malfunction could trigger the plane into a dive – as investigators believe happened with both the Lion Air and Ethiopian Airline crashes, killing 346.

The release of the document, in addition to warnings from Boeing’s chief test pilot that the MCAS system could engage without warning, adds to belief that Boeing overlooked safety in a rush to put the Max 737, its most profitable model, into production.

The new email is among others unearthed during withering transportation committee hearings during which lawmakers have repeatedly assailed Boeing’s CEO, Dennis Muilenburg, over a failure to prioritize safety.

“You have a systemic problem in your company. You’re driving profit. You’re not driving quality, and you’re sure as heck not driving safety,” the California Democrat John Garamendi told the chief executive.

In another memo, a veteran Boeing production manager warned the company was placing the workforce under too much strain in its rush to speed up production of the jet.

“Frankly right now, my internal warning bells are going off,” the manager wrote in the email, which was sent four months before the Indonesia crash. “And for the first time in my life, I’m sorry to say that I’m hesitant about putting my family on a Boeing airplane.”

The email, which was sent to the general manager of the Max program, warned that workers on the Max assembly line were exhausted by the schedule and there should be a temporary halt.

“I know this would take a lot of planning, but the alternative of rushing the build is far riskier,” the manager wrote.

At the hearing, Muilenburg confirmed the note’s author had copied the warning to him, and he had answered them. However, Boeing did not slow its rate of production.

Peter DeFazio, the Democratic chairman of the committee, told the hearing the manager had resigned his job at Boeing “because his concerns weren’t being addressed”.

He accused Boeing of a “lack of candor” over the crashes, after John Hamilton, Boeing’s chief engineer, testified that the company had identified MCAS as the probable cause within days of the first crash and begun to work on a fix.