How did horsemeat enter the food chain?
Britain's FSA said the evidence it had "points to either gross negligence or deliberate contamination in the food chain."
It said it was working closely with police, who would be involved if evidence suggested a level of criminality within the UK.
Environment Secretary Owen Paterson said the frozen burgers from Tesco and the lasagne from Findus were believed to be linked to suppliers in Ireland and France respectively.
Paterson said the French authorities viewed the issue "as a case of fraud rather than food safety."
In an oral statement to parliament on Monday February 11, Paterson said the "ultimate source of these incidents is still being investigated."
He said he had been in contact with ministers in Ireland, France and Romania and that the issue appeared to be one of "fraud and mislabeling."
AFP reported that Comigel had blamed French meat-processing company Spanghero, which blamed Romanian abattoirs where it said the meat was bought via traders in Cyprus and the Netherlands.
But Romania's prime minister said the two Romanian slaughterhouses initially suspected to have links to the horse meat scandal never had direct contact with Comigel and had not done anything illegal.
Minister of Agriculture Daniel Constantin said there was no evidence false horse meat labeling occurred in Romania.
What action are food authorities taking?
Britain's FSA has ordered food businesses to use independent laboratories to test all beef products for authenticity -- to see whether the content of the meat matches the label.
The deadline for the first round of testing is Friday February 15.
The FSA has also ordered Findus to test for bute, with results due "in the next few days" and to be published on the authority's website.
It has advised any retailers or producers that had sourced beef products from Comigel to conduct a precautionary withdrawal of product.
In France, consumer affairs minister, Benoit Hamon, has also ordered an immediate investigation and said results will be available by midweek.
In a statement, Hamon said a provider in Luxembourg and traders in Cyprus and the Netherlands were part of the chain being probed.
The Swedish National Food Agency has announced it is reporting Findus to police, which is the standard course of action when products have been sold with the wrong labels.
European Union officials plan to meet in Brussels to discuss the issue on Wednesday February 13.
How has the public reacted?
The revelations have revolted many meat eaters in the United Kingdom, where horse meat is generally considered taboo, although it is commonly eaten in neighboring France, as well as countries including China, Russia, Kazakhstan and Italy.
January's discovery of pig DNA in beef products is of particular concern to Jews and Muslims, whose dietary laws forbid the consumption of pork products.
Jewish dietary laws also ban the eating of horse meat.