Mexico has demanded answers from Washington following a report in a German weekly that US agents hacked into former president Felipe Calderon's email account.
Der Spiegel cited documents provided by former National Security Agency contractor Edward Snowden as the source of its report, which claimed that the NSA has spied on the Mexican government for years.
"The Mexican government reiterates its categorical condemnation of the violation of privacy of institutional communications and Mexican citizens," Mexico's foreign ministry said in a statement Sunday.
"This practices is unacceptable, illegitimate and contrary to Mexican law and international law," the statement read, adding that it will ask US officials for answers "as soon as possible."
In May 2010 the NSA hacked into the email service of the Mexican presidency, and were able to access president Calderon's email messages, according to Der Spiegel.
"In a relationship between neighbors and partners there is no room for the practices that allegedly took place," the Mexican statement read.
Snowden, who has taken refuge in Russia, is wanted in the United States for espionage and other charges after leaking details of the NSA's worldwide snooping activities.
In September, US journalist Glenn Greenwald, who has access to documents leaked by Snowden, reported that the spy agency hacked current Mexican President Enrique Pena Nieto's emails before his election last year.
Greenwald also told Brazil's Globo television that the NSA had monitored the online activities of Brazilian President Dilma Rousseff, as well as those of state-run energy giant Petrobras.
The disclosures led Rousseff to slam the United States in an address to the United Nations and to scrap a planned state visit to Washington.
In September Pena Nieto said that in a telephone call, Obama promised to investigate the spying allegations.