Kenyan police discovered a large quantity of explosives packed in boxes on a bus in the capital Nairobi that was headed to a town in the coastal region, a senior officer said on Wednesday.

It was not immediately clear who was behind the consignment – of 403 rolls of ammonium nitrate – but Kenya has suffered a series of grenade and gun attacks since October 2011 when its troops went into neighboring Somalia in pursuit of al Qaeda-linked insurgents it blames for kidnapping security personnel and Western tourists from its territory.

The sometimes deadly attacks, in Nairobi, on the Indian Ocean coast and regions bordering Somalia, have rattled investors and tourists in east Africa’s biggest economy.

Patrick Oduma, the officer in charge of police in Nairobi’s central business district, said the bus’s driver has been arrested and was being questioned over the explosives, which were originally offloaded from a bus that had arrived from neighboring Tanzania.

“The parcel of cartons disguised as books were destined to be transported to Voi,” Oduma told Reuters, referring to a town 142 km (92 miles) inland from the port city of Mombasa.

“The explosives, which are rolls of ammonium nitrate, can be used to bring down buildings,” he said.

Oduma declined to speculate on what the explosives might have been intended for. He said they had been handed to bomb experts and the Kenya Anti Terrorism Police.