Reports say the operation was approved by Defence Secretary Ben Wallace and Home Secretary Priti Patel on Sunday night.
Seven Nigerians who tried to hijack an oil tanker off the coast of the Isle of Wight, have been bailed.
UK media, however, say they are in the custody of the British border force.
After a ten-hour stand-off between the seven men who have not been named and the 22 crew members, 16 Special Boat Service commandos were sent to end the tension.
Reports say the operation was approved by Defence Secretary Ben Wallace and Home Secretary Priti Patel on Sunday night.
"Seven men have been bailed for their part in the maritime security incident that took place on board the Nave Andromeda off the coast of the Isle of Wight on Sunday, October 25," Hampshire police said in a statement on Thursday.
"The seven arrested men, who are all Nigerian nationals, have subsequently been bailed in relation to the criminal investigation whilst investigators continue their inquiries, but will now remain detained under Border Force powers."
The oil vessel which was sailing from Lagos to South Hampton had made a mayday call a few minutes after 11:00 am Nigerian time.
The captain of the crew, who is a Greek national, said they were under threat. Neither he nor his mates were injured.