"Our hearts are heavy, and we continue to extend our deepest sympathies to the loved ones of the passengers and crew on board," Muilenburg said in an open letter released by Boeing on Monday night.

"Safety is at the core of who we are at Boeing, and ensuring safe and reliable travel on our airplanes is an enduring value and our absolute commitment to everyone," Muilenburg added.

Muilenburg said the company was "taking actions to fully ensure the safety of the 737 MAX", adding that a software upgrade for its 737 MAX aircraft was coming "soon."

The open letter came after Muilenburg issued a statement on Ethiopian Airlines Flight 302 accident investigation on Sunday.

The 737 MAX jet has been grounded globally for safety concerns following the March 10 crash of an Ethiopian plane, which killed 158 people. The disaster followed a Lion Air crash in Indonesia in October of the same model, which killed 189 people.

The voice and data recorders, also known as black boxes, were sent to France.

Though the investigation into the causes of the crash are still in progress, French air accident investigators confirmed on Monday that flight recorder data from the Ethiopian Airlines jet showed "clear similarities" with the Lion Air jet.

In addition, the Wall Street Journal reported on Sunday that US Department of Transportation officials were probing into the aircraft's certification by the Federal Aviation Administration (FAA) and a grand jury in Washington subpoenaed at least one person involved in developing the MAX.

Boeing has halted delivery of all new MAX jets to its customers after the crash in Ethiopia.

After a 10 percent drop last week that wiped nearly $25 billion off its market share, Boeing stock closed at $372.28, down $6.71 or 1.77 percent on Monday, sharply lower than the 52-week high of $446.01 it reached at the beginning of March.