LAGOS, Dec. 11 (Xinhuanet) -- Investigators on Sunday began work to determine what brought a Nigerian passenger jet down killing 106 people, most of them children, the second major air disaster that hit Africa's most populous nation in two months.

    The Sosoliso Airlines flight traveling from Abuja, Nigeria's capital, on Saturday crashed and burst into flames on a final touch down to the runway in the southern oil city of Port Harcourt.

    Four of the seven survivors reportedly died in the hospital barely 24 hours later, bringing the death toll to 106. Dozens of the passengers killed are students, aged between 12 and 16, from an Abuja school returning home for the Christmas holidays.

    The Medicine Sans Frontiers (Doctors Without Border) said two of its employees, an American and Frenchman, were among the dead.

    Aviation officials originally said the DC-9 aircraft was carrying 110 people, but on Sunday, an official manifest issued bythe airline showed it had only 102 passengers and seven crew on board.

    The cause of the crash is not immediately available but Sam Adurogboye, spokesman for the Nigerian Civil Aviation Authority, told Xinhua that the plane ran into "bad weather." Local media said the crash was caused by a thunderstorm that struck when it was about to land.

    "Lightning struck abruptly, thus igniting fire. The impact of the thunderbolt caused the plane to disintegrate into two while most of the passengers killed were burnt," Nigeria's The Guardian newspaper said. Some passengers at the airport said they heard a loud explosion as the plane landed.

    The newspaper said "most of the passengers still had their seatbelts on" when they died.

    In Port Harcourt, Tomi Oyelade, permanent secretary in the Aviation Ministry, told reporters that US investigators will help determine the cause of the crash. "We have invited accident investigators from the United States of America who have over 27 years of experience in accident investigation," Oyelade said.

    "So far ... we saw the black box and flight recorder. We have handed over the crash site to the accident investigators," he said."We are told that the bad weather must have constituted but the investigators will confirm the actual cause."

    Jessy Markata, who took the same flight from Port Harcourt to Abuja early on Saturday before it crashed, said that the aircraft had developed faults on landing in the capital airport.

    "We could not land at Abuja for a very long time," Markata toldNTA state television, "I was surprised when the same aircraft was loaded back to Port Harcourt."

    The Sosoliso Airlines' headquarters in Lagos, Nigeria's commercial capital, were closed on Sunday while some relatives of the victims are waiting there in vain.

    "The Sosoliso officials would not talk to us, they are runaway,even the security men are runaway," said an angry middle-aged man who lost one of his family in the air crash.

    Sosoliso Airlines, established in 1994 as a wholly Nigerian owned company, was the 2004 best domestic airline of the year award winner in the west African country. It began scheduled flights as a domestic airline in July 2000 and now flies to six Nigerian cities, according to its website.

    Nigerian President Olusegun Obasanjo, who had vowed to "plug loopholes" in the country's aviation sector shortly after October's air crash that killed all 117 on board, canceled a scheduled state visit to Portugal on Sunday.

    A presidential statement said he will summon all airline operators to a crucial meeting with him on Tuesday. "It is expected that conclusions reached at the meeting will help to givemore practical effect to the president's avowed determination to carry out urgent and much needed reforms in Nigeria's aviation industry," the statement added.

    Nigeria has a poor record for aviation safety because most of its airliners are second ones having used for more than 20 years or even over 30 years.

    On October 22, a Boeing 737-200 operated by Nigeria's Bellview Airlines crashed on the outskirts of Lagos, killing all 117 peopleon board. But the black boxes are still missing and there are no official words on the cause of the air crash. Enditem