Barcelona announced an internal investigation Tuesday after reports in Spain claimed that the club had commissioned detective agencies to spy on a number of their star players.
The Catalan giants confirmed on their official website that the club spent a total of 3.1 million euros ($4.04 million) on "espionage" between 2008 and 2010.
The money went to the now defunct Metodo 3 agency and another company called the Intelligence Bureau and Cyber Experience.
Barcelona said that "legal action will be taken to preserve the image of the club, the privacy of its employees and the legacy of the institution."
Spokeman Toni Freixa added: "FC Barcelona wants to know 'who was spied on and why.' "
The alleged activities took place under the presidency of Joan Laporta, who was at the helm from 2003-2010, one of the most successful spells for the famous Spanish club.
Current president Sandro Rosell, a fierce critic of Laporta, succeeded him in June 2010, winning 60% of a vote among Barcelona club members.
The relationship between the two has remained cool since Rosell was elected, but on the pitch under former coach Pep Guardiola and his successor Tito Vilanova, the triumphs have continued.
Reports on the Spanish website El Confidencial, claimed Metodo 3 trailed defender Gerard Pique and his pop star wife Shakira in 2010 -- following them to a concert to log their activities.
Former stars Samuel Eto'o and Brazil's Ronaldinho were also believed to have been monitored in the past.