Some fellow truck drivers on social media raised questions about whether the driver would have known what was inside the trailer before he drove off. Using the hashtag #JusticeforMo, the driver Danny Nicholson wrote that he would often pick up “unaccompanied trailers” shipped to the United Kingdom without a cab.

“This arrest of this driver Mo, well it could have been me or any driver I know,” he wrote on Facebook.

Richard Burnett, the chief executive of Britain’s Road Haulage Association, said that a driver picking up a trailer from Europe in a British port would not typically open the container. It would have seals on it intended to remain intact until the point of delivery, he added.

For Mr. Burnett, many questions remain about how the bodies were discovered inside a container that should have been sealed, and whether Mr. Robinson himself called the authorities.

“Is this opportunistic trafficking? We don’t know. Is this organized crime? At this stage, we don’t know,” Mr. Burnett said. “But it kind of looks like it, the scale of this looks like organized crime.”

The registration of the truck should have been a major red flag for anyone in the trucking industry, he said. The tractor unit was registered in Bulgaria, but had not operated there since 2017, according to the country’s government.

“This is not illegal, but we would describe it as regulation avoidance and it’s absolutely frowned upon,” Mr. Burnett said. “This would kind of demonstrate that this is a business that doesn’t take safety and regulation seriously and is probably illicit in terms of how it goes about doing things.”