The crew of a tug were being quizzed by police last night as specialist officers carried out an inch-by-inch search of the boat which could take two days to complete.
The Tanzanian-registered Hamal was intercepted off the north-east coast by the UK Border Force vessel Valiant and the Royal Navy Type 23 frigate HMS Somerset.
It was escorted the 100 miles to Aberdeen as part of what the National Crime Agency (NCA) described as an “intelligence-led” operation.
The NCA declined to confirm or deny officers were searching for drugs on the Hamal – but at one point yesterday sniffers dogs were taken on board.
The tug’s nine crew were detained when it arrived in Aberdeen harbour and were being questioned last night.
The Hamal – which had been en route to Hamburg in northern Germany – was 100 miles east of the Aberdeenshire coast when the Navy and Border Agency swooped.
The boat will be held in Aberdeen harbour over the weekend.
The search of the vessel is being overseen by the NCA – the UK’s equivalent of the FBI.
Witnesses described seeing sniffer dogs arrive at the Point Law berth where the vessel was anchored yesterday.
A spokeswoman for the NCA would not reveal if they were searching for drugs – or what else they could expect to find.
She said specialist teams would remain at the harbour over the weekend as the investigation continued.
Representatives of Aberdeen harbour and police confirmed they were assisting with the operation.
“Acting on intelligence supplied by the NCA a vessel was boarded by a team from the Royal Navy frigate HMS Somerset and Border Force cutter Valiant about 100 miles east of the Aberdeenshire coast,” the NCA spokeswoman said.
“The vessel was then accompanied into Aberdeen where a full search is now being carried out by specialist Border Force officers with operational support from Police Scotland.
“Nine crew were detained for questioning by investigators from the NCA’s Border Policing Command.”
The Hamal was built in England in 1979, and has changed ownership numerous times over the past four decades.
During the past two months, the boat has berthed at the small fishing town of Tuzla in Turkey and at the Spanish holiday resort of Santa Cruz de Tenerife.
The latest tracking from maritime radars shows the tug was heading for Germany and was expected to reach Hamburg around 7pm last night.