Twenty-six people have been arrested in Belgium and France in an operation prompted by the deaths of 39 Vietnamese migrants in a lorry in the UK in 2019.
A UK report found the migrants all died from asphyxia and hypothermia.
Thirteen people, including Moroccans and Vietnamese, were held in Brussels and 13 more in Paris, authorities say.
Belgian prosecutors said the suspects had probably “transported up to several dozen people every day for several months”.
The smuggling network, they said, was suspected of having made the October 2019 trip possible.
The EU’s Agency for Criminal Justice Co-operation (Eurojust) said police had carried out cross-border raids on Tuesday morning in an operation that involved four nations – the UK, France, Belgium and Ireland – along with Europol.
A number of people had earlier been arrested in connection with the deaths, including several in Vietnam.
The driver of the lorry, Maurice Robinson, pleaded guilty last month to 39 counts of manslaughter.
At the same hearing at London’s Old Bailey, co-defendant Gheorghe Nica denied 39 counts of manslaughter. Another three men charged with other offences in connection with the deaths also appeared via video-link.
A trial on remaining charges is scheduled to begin on 5 October.
The bodies of the Vietnamese nationals were discovered at an industrial estate soon after the lorry arrived in the UK on a ferry from Zeebrugge in Belgium.
Among the men, women and children were 10 teenagers, two of them 15-year-old boys.
Among those who died was 26-year-old Pham Thi Tra My, who sent her family a message on 22 October saying she could not breathe and her “trip to a foreign land has failed”.