Nigeria ? Nigeria said Monday it had requested help from the United States to determine what caused an airliner to slam into the bush, killing all 117 people aboard and carving a pit deep into the earth.

Aviation Minister Babalola Borishad did not rule out foul play as the reason why the Bellview Airlines Boeing 737-200 went down late Saturday north of Lagos, but said: “For now, we just believe it’s an accident.”

Earlier, the National Civil Aviation Authority chief said a “natural cause” was likely.

Borishad said Nigeria had made a formal request for help from U.S. aviation investigators and forensic experts. U.S. Embassy officials had no immediate comment.

The flight lost contact with the control tower five minutes after taking off from the Murtala Muhammed International Airport in Lagos en route to the capital, Abuja, officials said. State radio said its pilots issued a distress call before the plane disappeared from the radar.

It crashed near Lissa, about 30 miles north of the airport in Lagos, Nigeria’s largest city. It was to be a 50-minute flight.

Nigeria announced a three-day, nationwide mourning period for victims.

The nationalities of those aboard were not immediately known, but most were believed to be Nigerians. Officials said 117 people were on board – 111 passengers and six crew members.