The crime in question is abuse of power or perverting the course of justice (prevaricación in Spanish), which is using one's power in his/her own favor. By law public servants must abstain from taking part in any decision affecting their own private interests, failing to do it is abuse of power, a very serious crime.
Incidentally all ministers of the Rajoy government have important financial investments, owning stock of various banks, etc. Their decisions in the Bankia scandal or in general on how to manage the Spanish public budget are probably affecting their private interests as owners of financial stock.
For that reason an association named Anticorruption League (Liga Anticorrupción) has denounced them before the Attorney General's Office.
Mariano Rajoy himself has €145,000 in equities, which gave him €8,000 in profits last year (5.4%). He also has €60,000 in productive banking accounts, €80,000 in savings' insurances, etc.
The Minister of Agriculture Miguel Arias Cañete is the wealthiest of the bunch, owning 4000 titles of Banco Santander, and 17,000 of BBVA, as well as investments in other financial entities totaling €72,000. He also owns stock valued in more than €300,000 of fuel companies like Petrolifa Ducar S.L. and Petrologis Canarias S.L.
Next to him is Minister of Work, the incredibly ugly (even for PP standards) Fátima Báñez owns €330,000 in Spanish bonds, as well as 80,000 titles of the family company Grupo Báñez S.L., whose value is not known but is being investigated for corruption in Seville.
The clon of Aznar, José Manuel Soria, Minister of Industry, has €270,000 in diverse stock, plus €50,000 in BBVA titles and €77,000 in titles of a mysterious Tarundanta company.
The Minister of the Treasury and Public Administrations, Cristobal Montoro, has three declared pension funds adding up to some €120,000.
Vice President Soraya Sáez de Santamaría (daughter of a well known fascist general) owns some €100,000 in hedge funds. Minister of Development, Ana María Pastor, has declared €65,000 in bank accounts.
The Minister of Justice, the sexist Alberto Ruiz Gallardón, who wants to destroy the right to abortion and used to be Mayor of Madrid before landing in this inquisitorial job, declared only €26,000 in a bank account (he lies for sure!)
The allegedly poor guys of this Posh Club that is the Spanish Government are Jorge Fernández, Minister of Interior (Police), and Ana Mato, Minister of Health Care, with only a few thousand euros in investments.
Three ministers, not being Members of Parliament, did not declare their wealth. They are Luis de Guindos (Economy), José Ignacio Wert (Education) and J.M García Margallo (Foreign Affairs).