Eleven people are being questioned by Dutch police as part of an inquiry into human smuggling after two people jumped from a North Sea ferry near Harwich, Essex, on Wednesday.
Two men, believed to be Albanian, went overboard from the Stena Britannica ferry.
They pair is believed to be part of gang of 13 who were refused entry to the UK at Harwich before being sent back on the ferry towards the Netherlands.
When the ferry arrived in the Netherlands only 11 of them – nine Albanians and two Vietnamese nationals - were accounted for by the Dutch authorities, according to the Border Force.
These 11 are now being questioned as part of an investigation into human smuggling.
A Border Force spokesman said: "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."
Robert Van Kapel, a spokesman for the Royal Dutch Police, said: "In England the immigrants were found stuffed in this freight vehicle and they were sent back to Holland where the boat came from.
"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."
Rescue teams have stepped down from the search but Suffolk coastguards are continuing to monitor the coastline for the missing pair.