MEXICO CITY (AP) — Former Mexican President Felipe Calderon says the reported spying on his presidential email system by the U.S. is an "insult" to Mexican institutions that should be addressed by current Mexico authorities.

Calderon says in a tweet Monday that he will closely follow the efforts by Mexico's Foreign Relations Department to get an explanation from the United States.

A report Sunday by the German news magazine Der Spiegel says documents from National Security Agency leaker Edward Snowden indicate the U.S. gained access to Calderon's email system when he was in office.

Mexico's government said Sunday it "categorically condemns" email spying. Opposition politicians are calling that response lukewarm.

Earlier, a document dated June 2012 indicated the NSA had read current Mexican President Enrique Pena Nieto's emails before he was elected.