In our series of letters from African journalists, Mannir Dan Ali, former editor-in-chief of Nigeria's Daily Trust newspaper, says the shooting down of a military jet shows how organised crime is becoming more daring by the day. Nigerians refer to them as bandits - a word that does not quite do justice to what are in fact networks of sophisticated criminals who operate across large swathes of northern-western and central Nigeria. Gangs on motorbikes terrorise the region, stealing animals, kidnapping for ransom, killing anyone who dares confront them and taxing farmers - it's a huge money-making operation. Over the last four years the security forces have not been able to get a handle on the situation, which millions of Nigerians feel is out of control. Last week President Muhammadu Buhari inaugurated the Dutsinma-to-Tsaskiya road in his home state of Katsina but few people dare travel on it after countless attacks. Most top government officials, including security chiefs, take the train linking the capital, Abuja, to Kaduna because of frequent abductions on the road between the two cities. A serving army general was recently killed on the main road from Abuja to central Kogi state and his sister, who had been travelling with him, was kidnapped. This week, 13 military police were killed in an ambush in Zamfara state when at the same time at least 150 villagers were abducted. At the moment at least 300 students are being held by kidnappers who seized them from their schools in Kaduna, Niger and Kebbi states at different times over the last two months - many taken in broad daylight. Some are Islamic primary school students as young as five and most of them, if the kidnappers are to be believed, have fallen sick.  In all these cases, the gangs are asking for huge amounts of money to release the children - ransoms the parents cannot afford, while the authorities insist that they will neither pay ransoms nor negotiate with criminals. The kidnappers, whose hideouts are in vast camps in forests, are brazen.   As they hold out for payment, they hassle parents with demands for bags of rice, beans and cooking oil to feed their captives.  Dozens of schools spread across at least five northern states have been closed by the authorities as they are unable to protect them.