Five men have been arrested after a group of migrants, including a child, were found inside a cattle truck.
The 13 migrants were discovered in a hay compartment of the truck carrying livestock in the French port of Calais on Saturday evening.
The British driver of the lorry was detained by French authorities.
Four men, aged 23 to 39, were arrested by the National Crime Agency (NCA) in a string of raids in Romford, London and Brentwood in Essex.
Investigators also searched properties in Grays, Essex, and Worthing, West Sussex, seizing about £100,000 in cash at one.
NCA regional head of investigation Gerry McLean said: "Those who seek to profit from facilitating illegal immigration into the United Kingdom do so for their own selfish greed and without any concern for the health and wellbeing of those who are often moved across borders in inhuman and degrading conditions.
"We are determined to do all we can with our law enforcement partners to disrupt those networks and bring perpetrators to justice."
The four men arrested in the UK on suspicion of facilitating immigration have been released under investigation, while the lorry driver remains in custody.