The ship is suspected of being involved in these incursions, which Denmark has called the "most serious attack" on its critical infrastructure to date. Russia has denied any involvement in this and other recent drone flights in Europe.

As of Wednesday, the Benin-flagged vessel -- named variously the Boracay, the Pushpa or the Kiwala -- had spent several days off the French coast. French forces boarded it and two crew members were taken into custody, officials said.

But from September 14 until September 20 it had been located off the coast of Russia, before crossing the Baltic Sea to the west.

Specialist website The Maritime Executive said it and other ships could have been involved in the drone incursion, either as launch platforms or as decoys.

Here is a breakdown of AFP's analysis of its movements.

- Days off Russian coast -

Data from VesselFinder showed the 244-metre-long (800-foot) tanker -- a "ghost tanker" sanctioned by the EU along with hundreds of ageing vessels used by Russia to circumvent oil export curbs -- left India in early August.

It remained moored off the Russian village of Ust-Luga until September 18.

Then it departed for the Russian port of Primorsk near Saint Petersburg, on the other side of the Gulf of Finland.

It remained at the oil terminal there for around 10 hours before heading west again.

- Drone flyover begins -

The drones began flying over Denmark on September 22.

That day, the data showed the tanker was off the coast of Poland and Sweden, then Denmark.

On September 23, at around 0300 GMT, it was spotted off the Danish island of Lolland, before sailing towards the Great Belt Strait, which runs along mainland Denmark.

By September 25, it was located some 160 kilometres (99 miles) west of Denmark.

- Heading south? -

By 1235 GMT that day it was steaming towards the English Channel, heading for the Atlantic to continue its journey southward.

Then, on September 28 at 0000 GMT, it changed course completely.

- Boarded in France -

It headed for the French coast -- but once it was in French waters the navy reported that the crew had failed to provide evidence of the nationality of the vessel and to cooperate with authorities, and a court probe was launched.

On Saturday French soldiers boarded the tanker, a military source told AFP. They were still on board, patrolling the deck on Wednesday, the source said.

Data from the Marine Traffic tracker showed the tanker was scheduled to arrive in Vadinar in northwestern India on October 20.