Mexico's government said Sunday it "categorically condemns" email spying, after a German news magazine reported that documents from National Security Agency leaker Edward Snowden say the U.S. gained access to the e-mail system of former Mexican President Felipe Calderon.

A report posted by Der Spiegel said the documents describe an operation dubbed "Flat liquid" that claim to have accessed Mexico's "presidencia" domain, which was also purportedly used by members of Calderon's Cabinet.

"This practice is unacceptable, illegitimate and contrary to Mexican law and international law," Mexico's Foreign Relations Department said in a statement.

It said it would send a diplomatic note to the United States. "In a relationship of neighbors and partners, there is no room for the kind of activities that allegedly took place," it said.

Harvard's Kennedy School of Government, where Calderon is now a fellow, said it would forward a request for comment to the former president.

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

Pena Nieto has said that would be an illegal act if it occurred, and his administration has demanded an investigation.

"By means of a diplomatic note, Mexico will re-emphasize the importance for our country of this investigation, which should be concluded as quickly as possible," the Foreign Relations Department said.