The U.S. National Security Agency hacked into a former Mexican president's public email account as part of a systematic, multiyear effort to eavesdrop on Mexican government officials, a German magazine reported on Sunday.
The weekly Der Spiegel ("The Mirror") cited a document provided by former U.S. intelligence contractor Edward Snowden that showed that a "top secret" National Security Agency operation "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 NSA by May 2010 had been able to tap into "diplomatic, economic and leadership communications which continue to provide insight into Mexico's political system and internal stability," according to the leaked document cited by Der Spiegel.
The document called the emails gathered during the so-called "Flatliquid" operation "a lucrative source."
Responding to the Der Spiegel report, an NSA spokesperson on Sunday said: "We are not going to comment publicly on every specific alleged intelligence activity, and as a matter of policy we have made clear that the United States gathers foreign intelligence of the type gathered by all nations."
"As the President said in his speech at the U.N. General Assembly, we've begun to review the way that we gather intelligence, so that we properly balance the legitimate security concerns of our citizens and allies with the privacy concerns that all people share," the spokesperson said.
The Mexican government on Sunday evening slammed the alleged eavesdropping, saying in a statement relayed by a Mexican Embassy spokesman in Washington that the digital surveillance is “unacceptable, unlawful and contrary to Mexican law and international law.”
“In a relationship between neighbors and partners (there is) no room (for) practices that allegedly took place,” the statement says.
According to reports, the alleged hacking into Calderon's account is not the first time U.S. intelligence services have eavesdropped on Mexican government officials. The NSA allegedly spied on sitting President Enrique Peña Nieto when he was seeking political office in 2012.
Related: Snowden leak journalist: NSA spied on emails of Brazil and Mexico presidents
Glenn Greenwald, the former Guardian journalist who along with filmmaker and activist Laura Poitras first published the contents of Snowden's leaks, told a Brazilian television program Sept. 1 that the NSA was privy to personal emails and text messages from Nieto.
Greenwald told the Globo news program "Fantastico" that a document dated June 2012 showed that Nieto's emails were being read one month before he was elected to office, and that the emails included information indicating who the then-newly-elected president would name to his Cabinet.
Amid Snowden's revelations, NSA officials have maintained that all alleged surveillance programs have operated within the confines of the law.
Snowden — who in August was granted temporarily asylum by Russia after fleeing the U.S. — is wanted by U.S. authorities for leaking classified information.
NBC News' Mary Murray and Henry Austin contributed to this report.