Image copyright NCA Image caption More than 20 AK47-style assault rifles were seized

Two men have been found guilty of their part in a plot to smuggle the biggest-ever cache of machine guns into the UK. Who was behind it?

Ringleader Harry Shilling, 26, from Swanley in Kent, and his right-hand man Michael Defraine, 30, from Bexleyheath in south-east London, tapped into an underground European arms market to smuggle 31 machine guns and more than 1,500 rounds of ammunition into the UK by boat, the MV Albernina, last August.

National Crime Agency investigators say the assault rifles - Czech-made VZ58s - were "mass casualty weapons" and the network that supplied the guns also supplied Amedy Coulibaly, the man who attacked a Jewish supermarket in Paris after his co-conspirators, the Kouachi brothers, stormed the office of the French satirical magazine Charlie Hebdo in January 2015.

The attacks killed 17 people over three days.

A former French paratrooper is one of four people arrested in connection with the supply of weapons to Coulibaly. Another Frenchman, a Serbian and a Montenegro national were arrested in Spain in April.

The NCA is still investigating who exactly supplied the arsenal to Shilling and his gang, but Rob Lewin, head of specialist operations at the NCA, says the cache seized was "four times the firepower used in the Paris attacks".

Image copyright Julia Quenzler Image caption Harry Shilling boasted of being a "gangster" in one text message

The former Eastern bloc military firearms, de-activated and modified to fire blanks, were sold legally in Slovakia, only for black market arms dealers to re-activate them and sell them on across Europe.

Slovakian police say more than 1,000 of the deactivated weapons were sold by a company called AFG, based in Partizanske, a provincial town 87 miles (140 km) north east of the Slovak capital Bratislava. The Slovak government tightened the law on how firearms are decommissioned to make it more difficult for them to be re-activated, six months after the Charlie Hebdo attacks.

Frantisek Gajdos, the shop's owner, says the legislation has decreased the sale of guns abroad "to zero" and there has been less interest in Slovakia too.

The AFG website sells a large array of assault rifles, machine pistols - like the nine Skorpion machine pistols found on Albernina - and handguns. A de-activated VZ58, which when modified is capable of firing 800 live rounds a minute, can be bought for 350 euros (£276). Once obtained and illegally re-activated the same firearm can be sold for between £2,000 and £4,000 to criminal gangs in the UK.

A sat-nav device used by the Shilling gang showed that on one of their trips to the continent they had travelled less than two hours drive from AFG's showroom, which specialises in de-commissioned weapons and has a sideline in Nazi uniforms and regalia.

Image copyright NCA Image caption Skorpion sub-machine guns were also among the haul

The investigation into Shilling - codenamed Operation Seventy - began in March 2015.

He was no stranger to criminality, although on a massively different level - he was fined once for criminal damage and on an earlier occasion he received a caution for the same offence. In 2007, when Shilling was in his teens, his father Michael was sentenced to 12 years for dealing in stolen truck parts.

The NCA's "armed operation unit" began investigating an organised criminal gang in Kent that they suspected had access to firearms.

The gang had links to a number of street gangs in London - the kind of criminals who would be eager to pay cash to buy or hire firearms.

Image copyright National Crime Agency Image caption Investigators from the NCA filmed the gang from the other side of the river

At first, investigators were unsure of Shilling's role, but as their work progressed they realised just how central he was.

Shilling made consistent attempts to cover his tracks. He was what the prosecution called "forensically aware".

The gang wore blue surgical gloves when they handled the weapons and unloaded their arsenal from the Albernina. Shilling changed his mobile phone five times during the investigation and communicated with Defraine via phones protected by encryption software.

Image caption The MV Albernina brought the guns into Kent from Boulogne in France last August

In a series of messages to an unidentified potential customer known as "B", investigators learned Shilling was willing to sell some of the weapons with ammunition, and he had plans to import more weapons for use by criminal gangs.

Exchanges between Shilling and Defraine revealed their excitement that the guns had safely been brought back from the continent.

Shilling told Defraine: "We are a firm ant we?" Defraine replied: "Proper heavy and armed to the teeth no one wants beef fam."

Shilling wanted to expand his fledgling criminal empire by becoming the UK's top armourer.

One of the gang had stored the number of one of Shilling's many phones under the name Kaiser. The prosecution said this was an in-joke and reference to Keyser Soze - the character from the cult crime movie The Usual Suspects.

In one of his encrypted messages Shilling boasted, "we now officially gangsters". When he was arrested he had a panic attack and had to be given first aid.

David Payne, 43, Richard Rye, 24, and Christopher Owen, 30, had all already pleaded guilty to smuggling the weapons into the UK. Payne and Rye also admitted the second count.

Two other defendants in the case, Jenny Arthy, 42, and John Smale, 58, were cleared of all charges.