Spanish newspaper El Periodico reported on Wednesday that Sanchez is accused of defrauding the public purse of €983,443 ($1 million) during 2012 and 2013 when he played for Barcelona.
"Alexis Sanchez has perfectly obeyed every one of the laws imposed by the countries where he has resided," Sanchez's agent Fernando Felicevich said in a statement.
"We ask the press and public at large for the upmost respect until the completion of the ongoing administrative and judicial process."
As in a litany of other cases involving players based in Spain in recent years, it is the income generated from Sanchez's image rights that has come under the spotlight.
Sanchez ceded his image rights to what prosecutors see as a merely instrumental company in Malta to avoid declaring tax on the rights in Spain.
Sanchez's former Barcelona teammates Lionel Messi and Javier Mascherano and Bayern Munich midfielder Xabi Alonso have also been investigated by the Spanish authorities for income derived from their image rights.
Messi, the five-time World Player of the Year, and his father were sentenced to 21 months in jail in July for tax fraud of €4.16 million.
The Messis have appealed the decision, but are unlikely to go to prison in any case as sentences of less than two years are normally suspended in Spain.
In a more similar case to Sanchez's, Mascherano accepted a suspended one-year prison sentence and a fine of 816,000 euros in January after admitting to tax fraud of €1.5 million.