Four days after they went missing in Niger, there have been no hard clues as to the fate or whereabouts of
Canadian UN special envoy Robert Fowler and his aide Louis Guay, also a former Canadian diplomat.
The West African state is among the world's poorest despite being a leading producer and exporter of uranium, used for nuclear fuel.
Tuareg rebels are fighting a campaign for autonomy in the north.
The United Nations and Niger's government believe Mr Fowler may have been abducted, although there has been no firm claim of responsibility. A rebel Tuareg splinter faction initially claimed the abduction on its website, but later retracted.
Here are some questions and answers about the case:
UN officials say the UN vehicle in which Mr Fowler, Mr Guay and a local driver were travelling was found abandoned and empty on Sunday evening on the south bank of the Niger River, near a crossing, some 45 km (30 miles) northeast of the capital Niamey.
The vehicle's engine was still running, its lights were on and personal possessions of the occupants were found inside.
Niger's foreign ministry said Mr Fowler had arrived in the country today and had left Niamey on Sunday morning without informing the Niger authorities. The government said it had no idea as to the motives of the disappearance, described by communication minister Mohamed Ben Omar as "inexplicable".
Niger security forces are searching for the missing men.