(Reuters) - More than 300 Boeing 737 MAX jets have been grounded worldwide after two fatal crashes in five months in Ethiopia and Indonesia killed nearly 350 people.

FILE PHOTO: Airplane engine parts are seen at the scene of the Ethiopian Airlines Flight ET 302 plane crash, near the town of Bishoftu, southeast of Addis Ababa, Ethiopia March 11, 2019. REUTERS/Tiksa Negeri/File Photo

Investigators looking to uncover the causes must answer one of the biggest questions: Was the plane’s software to blame?

WHAT WE KNOW

- Boeing has stopped delivery of all new MAX jets. Its stock has fallen about 6 percent since the Ethiopian Airlines crash on March 10.

- Boeing maintains its new, fuel-efficient jets are safe, but supported the U.S. Federal Aviation Administration (FAA) decision to ground them.

- The preliminary report into the Ethiopian disaster, published on April 4, showed a key sensor was wrecked, possibly by a bird strike. It began to feed faulty data into the Maneuvering Characteristics Augmentation System (MCAS), designed to prevent stalls by pushing the nose of the plane lower.

- Flying faster than recommended, the crew struggled with MCAS and turned it off. But the plane was still pointed down - making it hard to use manual controls. Data suggests they later turned MCAS-related systems back on - that would also reactivate the electric trim system to help push the plane higher. But with its power restored, a final MCAS nose-down command kicked in. Reactivating MCAS is contrary to advice issued by Boeing and regulators. The report did not address that. [nL8N21M5G9

- The preliminary report into October’s Lion Air crash in Indonesia suggested pilots also lost control after grappling with the MCAS software. It also focused on airline maintenance and training.

- Sources said on March 21 that Boeing would mandate on MAX jets a previously optional cockpit warning light, which might have warned of problems that possibly played a role in both crashes.

- Boeing said on April 3 it had successfully tested an update of the MCAS system.

- The U.S. Federal Aviation Administration said on April 3 it was launching a new review of the 737 MAX.

- Europe and Canada said on March 20 they would independently certify the safety of the jets, further complicating plans to get the aircraft back flying.

- Indonesia’s flag carrier Garuda said on March 22 it had sent a letter to Boeing asking to cancel an order for 49 MAX 8 aircraft, becoming the first airline to confirm plans to cancel an order after the crashes.

WHAT’S NEXT?

- A final report by Ethiopian authorities aided by air-safety experts from the United States and Europe is due to be published within a year.

- Indonesia has advanced the planned release of its report on the Lion Air crash to between July and August, versus a previous schedule of between August and September.

- U.S. lawmakers said on March 14 the 737 MAX could be grounded for weeks to upgrade software in every plane. Other countries may ground the planes even longer.

- The U.S. Transportation Department’s inspector general plans to audit the FAA’s certification of the jet, an official with the office said on March 19. The office can recommend changes or improvements to how the FAA operates.

- The U.S. Justice Department is also looking at the FAA’s oversight of Boeing, a person familiar with the matter has said. The FAA has said it is “absolutely” confident in its vetting.

- The chairman of the U.S. House of Representatives transportation committee and another key Democrat asked the Transportation Department’s inspector general to examine key decisions the FAA made in certifying the MAX jet.

- Ethiopian Airlines said on March 16 that DNA testing of passengers’ remains may take up to six months.