It is the biggest food fraud of the 21st century; it led to the withdrawal of tens of millions of burgers and beef products across Europe and a promise from David Cameron that everything possible would be done to get a grip on a "very shocking" crime. However, 10 months on, the details of how horsemeat came to adulterate large parts of the British and Irish food chain are still being kept from the public.

Now a Guardian investigation has unpicked one strand of this complex supply chain. Behind beef products sold by famous high-street names we have uncovered a labyrinth of murky meat brokerage that stretches across borders, and takes in drug and horse smuggling, animal welfare abuses, and one of Europe's richest beef tycoons.

At one point in the chain, there is testimony from migrant workers paid cash in hand to process defrosted meat that was "green" and years old. The Guardian has established that some meat from the plant was sent via a trader to the leading European supplier that manufactured adulterated beefburgers sold in several high street stores.

The shadow environment minister Barry Gardiner said that so long after the fraud came to light, it was a scandal that leading players in the industry had succeeded in shifting the blame and avoiding responsibility and that there had so far been no criminal proceedings. "The extraordinary thing is that because of its clout, industry has been able to commit what appears to be a criminal offence – selling the public horsemeat falsely labelled as beef – and just say they are sorry and didn't know. If every petty crook could get off by saying I didn't mean to and I didn't know, then our criminal justice system would be in a very sorry state."

One reason prosecution is so difficult is that retail supply chains have become so complex that pinning down the point at which the crime of mislabelling took place has proved difficult. The factory that supplied Tesco with its 29% horse "beefburgers", for example, was using "multiple ingredients from some 40 suppliers in production batches, and the mixture could vary in every half-hour", according to the Irish department of agriculture.

The Tesco burgers and those at Burger King, Co-op and Aldi that also tested positive for horse DNA were all made by the ABP group in its Silvercrest factory, in the border area of Ireland. ABP – the initials derive from Anglo-Irish Beef Processors – is the leading processor of cattle in Europe.

The company is owned by the Dundalk-born beef baron and property magnate Larry Goodman. The septuagenarian multimillionaire is famous for hard work, a love of private jets, high-level connections in the Irish government and keeping his business affairs secret. His business employs 2,500 people in the Republic and 8,000 in total in Britain, the Netherlands and Poland, in divisions which also include pet food, rendering and renewable energy from fat. About 50 million Europeans are thought to buy ABP products each week.

Goodman is no stranger to controversy. His companies were at the heart of allegations of fraud and political corruption that led to a public inquiry in Ireland in the early 1990s and helped bring down the Irish government. The report of the inquiry, known as the Beef Tribunal, found that there had been several incidents of fraud and an attempt to cover it up, as well as tax evasion. The judge also concluded that illegality in the company had involved the faking of documents, commissioning of bogus official stamps, passing off of inferior beef trimmings as higher grade meat and cheating of customs officers in the 1980s.

Goodman said he had been unaware of the illegal activity and blamed subcontractors operating without head office knowledge. The inquiry found no evidence that he had known.

Where did the horsemeat in burgers made by ABP come from? The Guardian has discovered that the company bought some of its meat from a Dutch businessman called Willy Selten via a trader who could have been a source. Selten ran a meat cutting plant in the town of Oss, south of Rotterdam. In May, he was arrested by the Dutch authorities on suspicion of fraud and false accounting, when official tests on boxes of meat labelled as beef taken from his factory found horse DNA in 21% of them.

Polish workers for Selten interviewed by the Guardian and Dutch TV have claimed they were cutting up and mixing horsemeat, delivered from slaughterers in the UK and Germany, with defrosted beef that was several years old – so old it was sometimes "green". They had to tie towels around their faces to stop themselves being sick. The workers described out-of-hours shifts paid in cash during which they mixed the different meats, and said relabelling of horsemeat had been going on for up to five years.

In the early days of the horsemeat investigation Selten was ordered to recall 50,000 tonnes of meat — enough for 100m packs of burgers — sold in the previous two years across Europe because he was unable to show where it came from.

Selten, who supplies numerous outlets around Europe, has denied fraud and false accounting. He told us through his lawyer that horse had been mixed with beef to order for only 10 months, and claimed that where old meat was being cleaned up and mixed with horsemeat he intended it for pet food.

ABP has blamed the adulteration in its chain on rogue managers at the Silvercrest site in County Monaghan, who, without the knowledge of head office, were buying in frozen meat for burger-making from traders who were not on the list of suppliers approved by its customers.

It admits they breached contracts and strayed out of supermarket and fast food specifications but insists no one in the company knowingly processed horsemeat.

The company told us meat from Selten was sourced for ABP not directly but on occasion by a Cheshire-based trading company called Norwest Foods. That company was set up by Ray MacSharry Jr, son of the former Irish agriculture minister and European commissioner and, the Guardian has established, a former employee of Goodman. Norwest's business interests include an abattoir in Spain, an office in Poland and an animal feed business in Ireland.

Last month both companies announced that they had reached a confidential financial settlement, and Norwest apologised for selling horsemeat to ABP Silvercrest unwittingly. ABP refused to answer questions about where the Selten meat had ended up.

So meat from Selten ended up at ABP for making burgers, but what were Selten's sources? The Guardian has managed to follow a trail from Selten's Dutch factory back to a key source of its horsemeat in the UK. Selten took deliveries from a Cheshire-based slaughterhouse known as Red Lion. It is owned by the Turner family, who slaughter and cut horsemeat and who own a cargo handling company in Dundalk, Ireland.

The Red Lion abattoir in Cheshire slaughtered horses and cut the meat. Photograph: Bruce Adams/Associated New/Rex

Before the horsemeat scandal broke, an animal sanctuary planted hidden cameras in key parts of the plant and filmed its Polish slaughtermen apparently abusing horses, and it became the subject of protests. By scouring data from campaigners about lorry movements from the abattoir, we were able to establish that it had delivered to the Dutch business. Polish workers at Selten's factory confirmed that Red Lion lorries arrived once a week. The Red Lion slaughtermen's licences were suspended and they are still being investigated by the authorities.

There is evidence that suggests horsemeat at the Red Lion abattoir came via a route involving organised crime. Two separate sources involved with enforcement have told us that Red Lion was the final destination of deliveries of animals from a loyalist Northern Irish horse dealer, Laurence McAllister.

He was tracked transporting unfit horses and donkeys, some without passports, from Northern Ireland via Scotland for disposal in the UK. His return load on one journey to Belfast was a quantity of cannabis worth more than £500,000, which was concealed in horse lorries. He was found guilty in October 2012 of drug smuggling and later of animal cruelty offences. Some of the horses he had been transporting were sick with chest infections, wounds, diarrhoea and sepsis.

A spokesman for the Turner family insisted that all their horsemeat deliveries to Selten had been properly labelled as horse and were legal. He at first denied but later acknowledged that one horse sent to Selten had been the subject of a recall having tested positive for 'bute', the horsemeat drug banned from the food chain. Tests for bute were taking three weeks at the time, so carcasses were often only recalled once they had already been sold on. The family through their lawyer, initially denied that horse had ever been bought by the abattoir from McAllister. Their lawyer later clarified the family's position, saying said that while they had never knowingly received horses from McAllister, it was possible they had bought from him indirectly without knowing. They had never knowingly slaughtered unfit horses or horses without proper passports, he added, pointing out that horses were only processed once they and their documents had been passed by official inspectors. There is no suggestion that the Turner family or employees at Red Lion had any involvement in drug smuggling.

Tesco, Burger King, Aldi and the Co-op have all apologised to customers and said they had been unwitting victims of fraud at some point in their supply chain. They also say the authorities have confirmed there was no food safety issue raised by the adulteration. They refused to answer the Guardian's questions about where the horsemeat in their beef products had originally come from, and whether Norwest or Selten were involved in their supply chains.

An ABP spokesperson said it had not engaged in any illegal activity and that there had been no breaches of law or food safety at Silvercrest. "We have made it clear we have never knowingly bought horsemeat … if equine was deliberately introduced into the food chain, then we are among those who have suffered as a result of such activity."