Barcelona's previous board, led by former president Josep Maria Bartomeu, showed "very serious criminal behaviour", a lawyer hired by the club said on Tuesday.
At the presentation of the club's 'forensic report' into the financial management of Barca under Bartomeu, current president Joan Laporta also said "payments without cause, payments with a false cause or disproportionate payments were found".
Laporta added that "it could not be ruled out that there was unfair reward for those people responsible for these payments".
The findings of the report prompted the club to file a complaint with the prosecutor's office in Barcelona last week. The prosecutor's office began an investigation into "economic crimes" on Friday, a source told AFP.
Barcelona's report, carried out by financial investigations company, Kroll, was initiated after the club's general director, Ferran Reverter, announced the results of an internal audit in October.
Reverter said the club was "technically bankrupt" when Laporta took over as president in March 2021, with the audit uncovering total club debts of 1.35 billion euros ($1.52 billion).
The presentation of the report was made in the offices outside Camp Nou on Tuesday morning by Laporta, Eduard Romeu, Barcelona's financial vice-president, and Jaume Campaner, a corporate lawyer contracted to work on the investigation.
Campaner said: "It's not about pointing fingers or describing the management of the previous board as better or worse, that's not it.
"It is about transferring information to the authorities that investigate crimes and clamp down on this sort of behaviour, which is very serious criminal behaviour."
Campaner continued: "If I had to define it all in one word, it would be: disloyalty. Because it is money from the members of FC Barcelona. That money cannot be abused or given away as if it was yours."
Campaner listed a number of alleged irregularities found in the report, including lawyers being paid seven million euros for the "supposed signing of a player", commissions paid to agents that were inflated from five to 33 per cent and a 15 million-euro payment made to a club to secure first refusal on young players, which Campaner said had "no basis in reality".
"The legal word for this is false accounting," Campaner concluded.
Bartomeu has defended his time as president, rejecting the figures presented by the current board while blaming losses on the crisis caused by the pandemic.
"Our management was serious and responsible," Bartomeu said in an interview with Barcelona newspaper Mundo Deportivo in October.
Laporta said on Tuesday he was still "open to dialogue" with Bartomeu. He said the club's wage bill had been reduced by 159 million euros under the current board.
Barcelona failed to secure an exit for Ousmane Dembele in the January transfer window, which closed on Monday night, with Dembele now able to leave for free when his contract expires in the summer.
Ferran Torres joined earlier in the month from Manchester City for 55 million euros while Laporta said Pierre-Emerick Aubameyang's signing on a free transfer from Arsenal should be completed this week.
Bartomeu appeared in court in March and was released pending corruption charges, after police carried out raids at five locations around Barcelona, including the offices at Camp Nou.
More than 200,000 Barcelona members had signed a petition to have Bartomeu removed as president but Bartomeu resigned in October 2020, before a vote of no confidence could take place.
