Nigeria's President Goodluck Jonathan has said there will never be a repeat of Sunday's fatal Lagos plane crash, promising improvements to air safety. He was speaking as he visited the crash site where rescue workers are still trying to retrieve bodies. All 153 people on board, and an unknown number on the ground, were killed. An official told the BBC investigators have recovered both the flight data and cockpit recorders, vital for understanding what caused the crash. 'Major setback' The McDonnell Douglas MD-83, operated by Dana Air, crashed into a printing works and residential buildings on Sunday afternoon before bursting into flames. "We have been working very hard to improve the aviation conditions in this country, so this particular incident is a major setback for us as a people," President Jonathan said as he toured the crash site just north of the airport. "I am here with members of the national assembly, because we will thoroughly investigate this... at the end of the day, we will make sure that this will never repeat itself again in this country," he said. Correspondents say this could be a difficult challenge in a country with a history of major passenger plane crashes - this is the fourth crash in the last decade in which more than 100 people were killed. But the African Airlines Association says Nigeria has made significant improvements in the last five years, "maintaining world class standards of safety". "Nigeria has been used as a model of what can happen if you have a government determined to improve safety - by giving autonomy to the civil aviation authority to do whatever is necessary to improve safety within the country," Elijah Chingosho, the secretary general of the association, told the BBC's Focus on Africa programme. "I hope the president will ensure that the investigation of the cause of this accident is thorough and then they can learn lessons from there which should be implemented as soon as possible and fully." Efforts to retrieve bodies and clear debris from the area continue as the country began three days of national mourning on Monday. 'Working tirelessly' The BBC's Sam Olukoya at the crash site says Sunday's scenes of chaos have been replaced by better co-ordination by rescue workers. "The search and rescue and recovery has been going on steadily. Today, a large number of bodies have been recovered. More are still in there, and they're working tirelessly to get them out," Tunji Oketunmbi, the spokesperson for Nigerian Accident Investigation Bureau, told the BBC. The cause of the crash remains unclear. Both the aeroplane's black boxes have been recovered, according to Mr Oketunmbi. "These are very vital to our investigation, they will help us lots in determining the causes of the accident," he said. Aviation Minister Stella Oduah said that the pilot contacted the Lagos control tower just before crashing to say the plane was experiencing trouble. Indian-owned Dana Air, which has yet to make an official statement about the cause of the crash, has set up a 24-hour hotline for relatives. "Our thoughts and prayers are with the families of guests who were involved in the Dana Air mishap. May the souls of the deceased rest in peace," its website says. Dana Air says it operates to cities around Nigeria out of Murtala Muhammed Airport in Lagos. The airport is a major hub for West Africa and saw 2.3 million passengers pass through it in 2009, according to the most recent statistics provided by the Federal Airports Authority of Nigeria.