The German magazine Der Spiegel reported Sunday it analyzed documents leaked by former NSA contractor Edward Snowden.
One top-secret document reported the NSA successfully breached high-level Mexican government communications in 2010.
"TAO [tailored access operations] successfully exploited a key mail server in the Mexican Presidencia domain within the Mexican Presidential network to gain first-ever access to President Felipe Calderon's public email account," the report said.
The Brazilian newspaper El Globo reported last month the NSA began monitoring the current Mexican president, Enrique Pena Nieto, when he was a candidate in 2012. The story led to Pena Nieto summoning the U.S. ambassador to lodge an official complaint.
The spying apparently began under Pena Nieto's predecessor, Felipe Calderon, Der Spiegel said.