A man and woman, both 38, residents of Warrington in Cheshire, North West England, were detained on suspicion of conspiracy to traffic people and manslaughter, Essex police said in a statement.
Earlier, Mo Robinson, a 25-year-old man from Northern Ireland was arrested on suspicion of murder following the discovery on Wednesday and remains in police custody, Efe news reported.
Essex police said the bodies of the victims, which were found in a container in the Waterglade Industrial Park in Grays, South East England, were on Friday being moved to a mortuary in Chelmsford and that postmortem examination would begin later in the day.
According to police, the tractor unit of the lorry travelled from Dublin and arrived to Holyhead Port on Sunday before picking up a trailer, which had come from Belgium, in Purfleet.
Belgium's Federal Prosecutor's Office said Thursday it had opened a case into the incident, saying in a statement an investigation showed the container concerned "arrived at Zeebrugge on 22 October at 02.49 pm and left the port the same day during the afternoon to arrive in Purfleet on 23 October at 01.00 am.
"It is not yet clear when the victims were placed in the container and whether this happened in Belgium," the prosecutor's statement added.
The suspect was in custody for questioning in a bid to establish the circumstances surrounding the incident.
The UK's National Crime Agency said it was working to identify criminal groups that could have been involved in the case.
Sources from Bulgaria's Foreign Ministry have said the trailer appeared to be registered in Bulgaria under the name of a company that belonged to an Irish national.
The case has generated a stir in the UK and reopened a debate about the dangers of migration mafias.
( With inputs from IANS )