Negreira has found himself at the centre of a firestorm of controversy that also involves Barcelona.
Between 2001 and 2018, the club made 33 payments to a company owned by Negreira, who was vice-president of the Technical Committee of Referees (CTA).
Barcelona have vowed to clear their name in court, having been charged with corruption.
Now, per El Mundo, the Civil Guard has ruled that, while Negreira was in his post, refereeing in the country was "not impartial".
They also claim that Barcelona "have not provided the identity of the professionals who would have developed the services" by Negreira's company, and have also failed to provide "copies of videos, documents or reports in which the specific advisory work is carried out".
The Civil Guard has stated that they believe allegations claiming that Barca paid over €8m (£7m/$8.6m) to Negreira are false.
The documents produced by the police force have been passed onto the Spanish courts, with Barca left to continue to sweat on the situation.
It remains to be seen how the case progresses.