Communal clashes, typically sparked by disputes over land, are common in Ethiopia. A fast-growing population of 105 million has added to pressures on farming in a country of sharp ethnic divisions.
Negasso, who was 75, was president between 1995 and 2001 and was the first head of state after Ethiopia adopted a new constitution
All 737 MAX aircraft have been banned from the world's skies since days after the Ethiopian Airlines crash on March 10.
Jet Airways has not been affected as its five 737 Max 8 aircraft have already been grounded - even before the grounding decision of the government -- due to non-payment of dues to lessors.
The head of Ethiopian Airlines said Thursday he was proud of the efforts of the pilots in trying to stop their jet from crashing, as he repeated condolences to the families of the 157 people who died.
The airliner went down soon after taking off on March 10, killing 157 people in the second deadly crash of a 737 MAX aircraft in less than five months, forcing a worldwide grounding of the model.
No details were provided about exactly when the report would be released, or what it might contain.
The findings were based on flight recorder data and represented the strongest indication yet that the system, known as MCAS, malfunctioned in both the Ethiopian Airlines crash and the Lion Air crash.
Boeing has flown test flights of its 737 MAX to evaluate a fix for the system targeted as a potential cause of the crashes, two sources familiar with the matter said Tuesday.
Ethiopian Airlines is widely seen as Africa's best-managed airline. It had been using five of the Max planes and was awaiting delivery of 25 more.
Flight ET 302 crashed on March 10 just minutes into its flight to Nairobi.
The accidents on Lion Air in Indonesia and Ethiopian Airlines, which killed 346 people between them, have raised major concerns about the safety certification of the 737 MAX 8 model.
The grounding of aircraft by crisis-hit Jet Airways, grounding of Boeing MAX 737 planes by SpiceJet and curtailed schedule of IndiGo, among other factors, have impacted flight schedules.
Chao's request comes amid reports US authorities have also launched a criminal probe into the certification of the top-selling jet, which has been grounded around the world.
Industry sources caution that the MCAS upgrade does not point to a cause of the Ethiopian crash, something Muilenburg's statement also indicated.