Castro said in court records that many company expenses that appear personal in nature were never declared on the couple's income tax returns, and that he must determine whether the amounts spent reach an annual value of more than 120-thousand euros (163,630 US dollars) that could turn the non-reporting into a crime punishable by prison.
If the amounts are less, the likely result would be an administrative tax investigation and possible fines.
The case stems from another one also led by Castro, in which he's investigating Urdangarin for allegedly using his Duke of Palma title to embezzle public contracts through the Noos Institute, a non-profit foundation he and a business partner set up that channelled money to other businesses, including Aizoon.
Court documents show Castro wants to clear up uncertainty about 1.2 million euros (1.6 million US dollars) that may have been transferred from Noos to Aizoon.
The 48-year-old princess, a bank foundation director, has been cleared of involvement in Noos.
The two cases are being investigated in Palma de Mallorca because many of the deals Urdangarin and a business partner brokered for Noos were for the
Balearic Islands and Palma de Mallorca is the largest city in the archipelago.
Latest World News
