A people smuggling plot that saw one man jailed could have involved a wider criminal network, the BBC has found.
Six Ukrainians were found on a yacht which ran aground near Orford, Suffolk, in April. Skipper Hennadii Kurtoglu was jailed for four years in July.
But further evidence found by the BBC suggests at least two other people could have been involved.
The Home Office said it was aware larger organised crime groups were often behind similar incidents.
The BBC traced the boat used in the smuggling plot to a boatyard in Nauerna, Holland and spoke to Peter Post, who sold the boat to Kurtoglu for €4,800 (£4,278), not knowing he was a people smuggler.
Ukrainian national Kurtoglu, 48, later pleaded guilty at Ipswich Crown Court to assisting unlawful entry into the European Union. The six people found on the yacht were deported.
Mr Post said two other people were involved in the sale - a younger man aged in his 20s and another man who phoned from London and called himself Peter Millar.
The men told him the boat would be used for a trip to Spain but he was suspicious that might have been smuggling drugs.
"But trafficking people? No, that didn't come up in my mind," he said.
He said neither the UK Border Force nor the Dutch equivalent had been in touch to ask about the men or follow up any leads.
Tony Smith, a former director of the UK Border Force, said he believed it was important "that all avenues of inquiry are followed through in cases like this, to ensure smuggling and trafficking gangs are properly dismantled and disrupted".
The Home Office said a "thorough investigation" had been conducted into Kurtoglu's people smuggling attempt.
"We recognise that in many cases people smugglers do not work in isolation and that there are often larger organised crime groups operating in the background.
"As such, it is not unusual for our investigations, or those of law enforcement partners like the National Crime Agency, to continue after initial convictions have been secured," a spokesman said.
During Kurtoglu's sentencing, Judge Martyn Levett described East Anglia as "a favourite target to land illegal entrants".
"There is probably about 200 miles of unpoliced coastline in Suffolk to choose from," he said.
The National Crime Agency said it was aware of the incident involving Kurtoglu and supported Immigration Enforcement "where required through the investigation".
It also said it would continue investigations "long after convictions have been obtained".
- BBC Inside Out East will have more on this story on Monday 6 November at 19:30 GMT on BBC One.