British police have raided three properties in Northern Ireland amid the investigation.
The truck and the trailer with the people inside apparently took separate circuitous journeys before ending up at the industrial park.
Detectives said the trailer containing the victims arrived at the port of Purfleet from Zeebrugge, Belgium, around 12:30 a.m. local time on Wednesday. The front section to which it was attached entered the country via Holyhead, Wales, on Sunday from Dublin.
Both the cab and trailer left Purfleet shortly after 1:05 a.m. Wednesday, and police were called less than an hour later after the bodies were discovered at the industrial park.
Police originally said they believed the truck may be from Bulgaria. The country’s foreign ministry confirmed the truck was registered in Bulgaria under a company owned by an Irish citizen.
In Belgium, authorities say they have made little progress in tracking the truck’s route.
Prosecution spokesman Eric Van Duyse said Thursday that “up till now, we have a lot of questions and not a lot of answers. We don’t even know which road was followed by the truck in Belgium.”
Van Duyse said authorities also do not know how long the truck was in the country, whether it stopped and whether people entered the container there.
Irish authorities have said they are also cooperating with the investigation and probing the registrations and movements of the refrigerated container and the Irish-owned truck.
Robinson has not yet been charged with any offense, and Essex Police, who typically have 48 hours to question a suspect arrested on suspicion of murder, were granted an extension to keep him in custody for an additional day.
“This is an incredibly sensitive and high-profile investigation, and we are working swiftly to gather as full a picture as possible as to how these people lost their lives,” said Deputy Chief Constable Pippa Mills.
“Our recovery of the bodies is ongoing and the post-mortem and identification processes, which will be lengthy and complex, can then begin,” Mills added.
The man’s parents have flown to England to support their son following his arrest. Paul Berry, a local politician in the county of Armagh who said he’s spoken with the Robinson family, said the village of Laurelvale where Robinson’s parents live was in “complete shock.”
“The local community is hoping that (Robinson) has been caught up innocently in this matter but that’s in the hands of Essex Police, and we will leave it in their professional hands to try to catch the perpetrators of this,” Berry said.
The truck was driven from the industrial site Wednesday evening to a secure area of the Port of Tilbury, where Thursday, police are expected to remove the bodies.
In 2000, a truck with 58 migrants who had died was found in Dover, England, and in 2015, 71 migrants from Syria, Iraq and Afghanistan were found dead in a truck in Austria.
A National Crime Agency assessment report on serious and organized crime last year said there was a “greater focus” on rising smuggler numbers in Belgium after the closure of the Dunkirk migrant camp in 2017.