New York (CNN Business)Boeing's CEO on Monday said the safety systems on its 737 Max jets were properly designed, but he added that the airline is working to make them safer following two recent deadly crashes.

Dennis Muilenburg said the pilots did not "completely" follow the procedures that Boeing had outlined to prevent the kind of malfunction that probably caused a March 10 crash of an Ethiopian Airlines jet. A Lion Air 737 Max crashed under similar circumstances in October.

The company's anti-stall software, called MCAS, was a common link in both crashes. It met Boeing's design and safety criteria, and adhered to certification protocols, Muilenburg told reporters following Boeing's annual shareholder meeting in Chicago.

"When we design these systems, understand that these airplanes are flown in the hands of pilots," he said. He added that Boeing was unable to find any "technical slip or gap" in building its MCAS software.