Ethiopian Airlines says searchers have recovered both flight recorders from the plane that crashed Sunday, killing all 157 people onboard.
Ethiopia is observing a day of mourning for those who died when the Boeing 737 MAX-8 jet, headed for Kenya's capital, Nairobi, went down just six minutes after takeoff from Addis Ababa.
At least 20 of the passengers were headed to the annual assembly of the U.N. Environment Program (UNEP) at its global headquarters in Nairobi. The U.N. flag was at half-staff when the delegates arrived at the meeting Monday.
Moments of silence were observed at the start of the Nairobi gathering and at the opening of a U.N. General Assembly meeting in New York.
"A global tragedy has hit close to home and the United Nations is united in grief. I extend my deepest condolences to the families and loved ones of all the victims, to the government and people of Ethiopia, and all these affected by this disaster," Secretary General Antonio Guterres said in New York.
Two witnesses say they saw smoke coming from the back of the plane before the crash, according to the Reuters news agency.
Co-pilot's brother mourns
Menur Nur Mohamed lost his brother Ahmed on the doomed plane. Ahmed Nur Mohamed was the co-pilot.
"Me and my brother grew up together. He wasn't only my brother, but also my friend," Mohamed told Tsion Tadesse of VOA's Horn of Africa service.
Mohamed said he learned of his brother's death when the head of Ethiopian Airlines mentioned his name.
"[My brother] called me Saturday night to tell me has a flight Sunday morning and will call me when he gets back. But I didn't know where he was flying. When I heard about the accident Sunday morning, I called his friends to find out where he flew. I was trying to calm down our parents when the CEO of Ethiopian Airlines gave the press conference," Mohamed said.
737 Max 8 jets grounded
Ethiopian Airlines has grounded its fleet of Boeing 737-Max 8 jets, following the deadly crash.
Boeing said in a statement late Sunday, "A Boeing technical team will be travelling to the crash site to provide technical assistance under the direction of the Ethiopia Accident Investigation Bureau and U.S. National Transportation Safety Board."
The plane's pilot had been given clearance to return to the airport after experiencing some difficulties, Tewolde GebreMariam, the head of Ethiopian Airlines said.
The Ethiopian Airlines CEO said he could not rule out anything as the cause of Sunday's crash.
"Ethiopian Airlines is one of the safest airlines in the world," GebreMariam told reporters while visiting the crash site.
He stood inside the crater where the airliner went down with the ground littered with plane parts and body bags.
China, Indonesia, and the Cayman Islands have grounded their Boeing 737-Max 8 jets.
Citizens from at least 35 countries were on the plane.
The Ethiopian Airlines jet went down in clear weather near the city of Bishoftu.
The Boeing 737-MAX 8 was a new jet, delivered to the airline in November, according to Planespotters, a civil aviation database.
Flightradar24, which tracks planes in real-time, posted on Twitter that the "vertical speed" of the Ethiopian aircraft "was unstable after takeoff."
Same plane as Indonesia disaster
The Boeing 737-MAX 8 is the same model that took off in October from Jakarta and crashed into the Java Sea a few minutes later, killing all 189 people on board the Lion Air flight.
Investigators with Indonesia's National Transport Safety Committee issued a preliminary report on that disaster in November.
The report was based on information from the flight data recorder, and said the plane's automatic safety system repeatedly pushed the plane's nose downward despite the pilots' desperate attempts to maintain control.
They believe the automated system that prevents the plane from stalling if it flies too high on Boeing's new version of its legendary passenger jet received faulty information from sensors on the fuselage.
WATCH: US Experts to Help With Plane Crash Probe
The plane had a similar problem on a flight from the resort island of Bali to Jakarta the night before the fatal crash. The investigators said the plane was not airworthy and should have been grounded after that flight.
Boeing had planned to unveil its latest 777x widebody Wednesday in Seattle, but has cancelled the debut to instead focus on "supporting" Ethiopian Airlines, according to Reuters news agency.
Tsion Tadesse, Peter Clottey, Margaret Besheer contributed to this report.