Eleven people are in custody in the Netherlands as part of a "human smuggling" investigation after two people reportedly jumped from a North Sea ferry.
The pair, thought to be Albanians refused entry to the UK, went overboard from the Stena Britannica ferry off Harwich, Essex, on Wednesday.
Rescue teams were stood down after no sign of the pair was found.
Essex Police are now heading a missing persons inquiry.
The pair were reported to have jumped from the vessel at about 09:45 GMT.
The ferry was later cleared to continue its journey to the Hook of Holland where 11 people are now in custody.
'Breaching immigration rules'
A spokesman for Border Force said 13 people who had arrived on a ferry at Harwich were refused entry to the UK by Border Force officers.
He said they had been on a lorry whose Lithuanian driver was arrested on suspicion of "facilitating a breach of UK immigration rules" but was later released without charge.
"They were returned to the Netherlands on a later ferry," he said.
"Upon its arrival in the Netherlands only 11 of the 13 were accounted for by the Dutch authorities.
"We are aware of reports that two people jumped from the ferry after it departed Harwich and Border Force assisted police and the coastguard with their search operation.
"The same vessel returned to the UK this morning and Border Force officers carried out extra checks on vehicles as they arrived. Two Albanian nationals remain unaccounted for."
'Human smuggling organisation'
Robert Van Kapel, a spokesman for the Dutch border police, the Royal Netherlands Marechausee, said nine Albanians and two Vietnamese nationals were being questioned as part of an inquiry into "human smuggling".
He said they had been found in a freight vehicle trying to enter the UK.
"In England the immigrants were found stuffed in this freight vehicle and they were sent back to Holland where the boat came from," he said.
"One of the main questions we have is 'Is there a human smuggling organisation active at this time?' That's what we are trying to find out."
He said it was "unusual" that two people had jumped overboard, and that such a large number of people had been found in one vehicle.
"We hope to find out how they got in, how much they paid and who they paid the money to," he said.