The boss of Boeing has denied accusations that its two 737 Max aircraft involved in fatal crashes lacked an optional safety feature, which might have alerted the pilots to technical malfunctions that partly caused the accidents.

“We don’t make safety features optional,” Dennis Muilenburg, Boeing’s chairman and chief executive, said at the company’s annual meeting in Chicago on Monday. “Every one of our airplanes includes all of the safety features necessary for safe flight.”

A preliminary investigation into the Ethiopian Airlines 737 Max crash last month found it was triggered by a faulty “angle of attack” sensor, which monitors the inflight position of the plane. The erroneous readings from the sensor in turn activated the aircraft’s maneuvering characteristics augmentation system (MCAS), an auto-pilot program that encourages the plane’s nose to dip down.

The crash killed 157 passengers and crew, months after another 737 Max operated by Lion Air crashed off the coast of Indonesia in similar circumstances with the loss of 189 lives.

It was revealed over the weekend that Boeing had removed warnings about pitch sensor malfunction from the standard 737 Max (MCAS) safety package. The Wall Street Journal reported that the warning system, which was present in previous 737 models, was only operative on 737 Max jets if the operator airlines had paid for a package of additional safety features.

WATCH: Boeing CEO Dennis Muilenburg holds a press conference, addressing 737 Max concerns. https://t.co/MKRgUkxGTg — CNBC (@CNBC) April 29, 2019

“In this case again, as in most accidents, there are a chain of events that occurred. It is not correct to attribute that to any single item,” Muilenburg said. “We know that there are some improvements that we can make to MCAS and we will make those improvements.”

He told shareholders that software updates being carried out would make the grounded 737 Max fleet “one of the safest airplanes ever to fly”, adding: “Yet, we know we can always be better. We have a responsibility to design, build and support the safest airplanes in the sky. The recent accidents have only intensified our dedication to it.”

Protesters, including some relatives of crash victims, carried placards outside the meeting calling on Muilenburg to be prosecuted for corporate manslaughter. They also carried large photographs of some of those who died in the crashes.

Friends and family of Samya Stumo, one of the victims in the Ethiopian crash, held placards reading: “Prosecute Boeing and Executives for Manslaughter.” Among the other placards were signs reading “Boeing’s arrogance kills”, “Prosecute Boeing & execs for manslaughter” and “Boeing is making a killing from killing”.

Muilenburg dismissed calls for him to resign, saying: “My clear intent is to continue to lead.” In his first media appearance since the the international grounding of the 737 Max, he said he was confident that Boeing would regain the trust of regulators and passengers. “We know we have work to do to earn and re-earn that trust,” he said.

At the start of the meeting, Muilenburg called on attendees to observe a moment’s silence to honour the victims of the crashes and said the accidents continued “to weigh heavily on us”.

More than third of shareholders voted for a motion to strip Muilenburg to of the Boeing chairmanship. The investor advisory firm Institutional Shareholder Services had advised its clients to vote for the motion, and for an independent chairman to be appointed to help the company rebuild its reputation.

It was also revealed on Monday that four Boeing employees had called the Federal Aviation Administration to raise serious concerns about the 737 Max. The calls began coming in within hours of Ethiopian investigators releasing a preliminary report on the crash. The calls from current and former Boeing employees allege possible issues related to the angle of attack (AOA) sensor and the MCAS anti-stall system that relies on data from the sensor, according to CNN.