The American captain of a packed passenger jet that crashed into an apartment building in Nigeria's largest city called the control tower to report engine failure just minutes before the plane went down Sunday, killing all 153 people on board, according to Nigerian aviation officials.
"The important progress we made in the investigation today was the discovery of the two black boxes in the wreckage, including the cockpit voice recorder," said Kunji Okecunbi, spokesman for the West African country's Accident Investigations Bureau.
The MD-83 plane of Dana Air crashed into a building in a densely populated neighborhood in Lagos, Nigeria, near the Lagos' Murtala Muhammaed International Airport on Sunday.
"The fear is that since it happened in a residential area, there may have been many people killed," said Yushau Shuaib, a spokesman for Nigeria's National Emergency Management Agency.
Okecunbi spoke to ABC News on his cell phone from the still smoldering scene of the wreckage site, where he said he had to cover his nose at times to block the smoke blowing his direction. Thousands of people were reportedly drawn to the crash, many to help any survivors out of the smoke and burning debris.
The Dana Air plane was reportedly flying from Abuja, the nation's capital, to Lagos when the accident occurred.
On the first of three official days of mourning in Nigeria, rescue workers used cadaver dogs and cranes to recover bodies. Officials do not yet know how many people were killed on the ground when the plane careened through a church, a textbook printing warehouse, and a two-story apartment complex. Local media report at least 10 casualties on the ground which would bring the death toll to at least 163.
For a second day, large crowds gathered around the crash site. At one point police used tear gas to keep the crowd from interfering with rescuers' work. Many of the onlookers were people hoping to locate missing relatives.
Nigerian President Goodluck Jonathan visited the site of his country's worst aviation disaster in 20 years where he vowed to further improve airline safety.
"This particular incident is a major setback for us… By the end of the day, I will make sure that this will not repeat itself in the country," Jonathan told reporters.
After a spate of commercial airline crashes in 2005, Nigeria made major aviation safety reforms. Sunday's crash was the first involving a commercial airline since 2006.
"The UN body that sets up global standards for aviation safety, the International Civil Aviation Authority, does audits of member states. Audits of Nigeria are now quite well above the global average," said Tom Kok, director of AviAssist, a non-profit working to improve aviation safety in Africa.
"No matter how safe a country's track record is there is always a chance for accidents of course. It is fair to say that whereas before 2005 you would say, 'Oh, it's Nigeria again,' but you would not say that now," said Kok.
The cause of Sunday's crash is still unknown. Officials have not released more information about the American captain of the plane or his Indian co-pilot. There are reports of other Americans on the flight but the U.S. State Department has not yet confirmed how many were on board.
At the time the aircraft went down, the weather was overcast but there were no storms. Authorities are investigating.
ABC News Radio and The Associated Press contributed to this report.