La Guardia Civil said the investigation began in May following an alert by the Agencia Española de Consumo, Seguridad Alimentaria y Nutrición (AECOSAN).
Those involved have different degrees of responsibility in the poisoning of 105 people by histamine after consumption of tuna, it added.
They are being investigated for crimes against public health with foodstuffs, falsification of documents and against consumer rights.
SCIRI and RASFF alerts
The AECOSAN alert was made in the Sistema Coordinado de Intercambio Rápido de Información (SCIRI) and also through the Rapid Alert System for Food and Feed (RASFF).
Tuna was marketed by Garciden, based in Almeria . The company did not respond to our request for comment when contacted earlier this year.
In Spain, Andalucía, Murcia, Valencia, Cataluña, Aragón, Castilla and León, Madrid and the Basque Country were affected.
We previously revealed that Italy, France, Croatia, Spain, Portugal, Poland and Denmark made notifications in the RASFF portal.
Histamine forms when certain fish are not properly refrigerated before being cooked or processed. Cooking, freezing and canning will not destroy the toxin after it has formed.
Symptoms include tingling or burning of the mouth or throat, rash, headache, diarrhoea and usually start within one hour after eating.
Guardia Civil findings
Initially two lots were involved and were withdrawn from the market.
Guardia Civil inspections at the company found the data and documentation provided in relation to the two lots did not match the documentation, delivery notes, invoices and labels found during other inspections and there were actually up to six lots from several suppliers involved.
Labels did not always match the batch number that was on the bill or invoice.
It was found the company provided some buyers with a copy of the analytical results on the state of the tuna but these had been ‘manipulated’ from the original.
Examining the labels, the Guardia Civil verified that information to consumers was not true as regards the catch areas of tuna.
Investigations to determine the exact moment of the cold chain problem with the tuna have been inconclusive due to the lack of documentary control in the company's traceability.
The European Food Safety Authority (EFSA) is in the process of a Rapid Outbreak Assessment following increasing RASFF notifications on histamine intoxication and cases from some countries.
When the assessment will be published is not clear but a deadline is set for the end of this month.