CHICAGO (CBS) — Resisting calls from some shareholders to resign over the 737 Max scandal, Boeing CEO Dennis Muilenburg said he’s focused on safety as the company continues working on a software fix for the grounded planes, after two fatal crashes resulted in more than 300 deaths.

At the first shareholders’ meeting since two fatal crashes in Indonesia and Ethiopia, Muilenburg tried to convince investors and the public that 737 Max aircraft are safe to fly again.

Officials believe, in both deadly crashes, faulty information from a sensor caused the anti-stall software on the 737 Max jets to push the nose of the planes down. Pilots followed Boeing’s procedures for that situation, but couldn’t regain control of their aircraft.

Muilenburg said Boeing is close to completing an upgrade to flight software on those jets to make sure nothing like that happens again.

“Our talented test pilots have made 146 737 Max flights totaling roughly 246 hours of air time with the updated software, and nearly 90 percent of our 50-plus Max operators around the globe have experienced that software update themselves during one of our simulator sessions,” he said at Monday’s shareholder meeting at the Field Museum.

Muilenburg made a hefty promise to shareholders, vowing the upgraded 737 Max planes will be among the safest aircraft ever to fly.

It was a tough crowd for Muilenburg, as some shareholders have called for him to resign, and others have filed a lawsuit accusing the company of defrauding investors by not disclosing safety issues on the 737 Max.

Regulators around the world grounded all 737 Max planes last month, in the wake of two deadly crashes in five months. A Lion Air flight crashed into the Java Sea off Indonesia in October, and an Ethiopian Airlines flight crashed outside Addis Ababa in March. A total of 345 people were killed in the crash.

Boeing is facing mounting lawsuits from the victims’ families, and a handful of relatives and friends of crash victims showed up outside the shareholders’ meeting in protest.

“They need to drop their charade of pretending that they’re all about safety and they’re just going to get better; and they need to come clean about the chain of events inside Boeing and the FAA that led to these crashes that killed 346 people,” said Tarek Milleron, the uncle of a crash victim.

On Monday, attorneys in Chicago plan to announce another wrongful death lawsuit against Boeing, on behalf of 10 Canadians killed in the crash in Ethiopia.

The U.S. Department of Transportation is forming a committee of experts to conduct an independent review of the FAA’s certification process for the 737 Max, after the Justice Department and Congress launched separate investigations.

The FAA reportedly handed off some or all of the approval process to Boeing itself, and pushed safety engineers to speedily approve the analysis of a new flight control system installed on the planes, known as the Maneuvering Characteristics Augmentation System (MCAS). The MCAS system was put on the Max jets to help avoid mid-air stalls but it has been linked to both deadly crashes.

Southwest Airlines also has said Boeing did not disclose it had deactivated a safety feature on its 737 Max jets until after the deadly Lion Air crash in Indonesia last October. A sensor known as an “angle of attack” indicator alerts pilots if a sensor is providing bad data about the pitch of the plane’s nose. Those sensors had been operational in previous versions of 737 planes, but were switched off on the 737 Max.

According to Southwest, the safety feature was “depicted to us by Boeing as operable on all Max aircraft.” Only after a Lion Air 737 Max crashed in Indonesia last Oct. 29 did Boeing say the feature wasn’t turned on, Southwest said.