According to German media reports on Friday, the German foreign intelligence agency Bundesnachrichtendienst (BND) intercepted at least one call during Clinton's time in office as US Secretary of State.
German newspaper Süddeutsche Zeitung and German regional public broadcasters NDR and WDR said that Clinton's calls had been hacked while she was on a plane. The location and date have not been disclosed.
German government sources have defended the data interception, saying that the call was picked up "by accident."
Another source told Süddeutsche Zeitung that the fact the call recording hadn't been destroyed immediately was a case of "idiocy."
Following a period of joint investigative journalism, the German media sources also discovered that the BND was ordered by the German government to spy on an unnamed NATO partner state.
The reports come just months after US-German relations hit rocky ground when the US National Security Agency (NSA) faced a chain of spying allegations.
Among the allegations were that the NSA not only conducted mass surveillance on German citizens, but also hacked into German Chancellor Angela Merkel's cell phone.
In May 2014, a 31-year-old BND employee admitted to passing on more than 200 documents to US intelligence agencies in return for a total of 25,000 euros ($34,000) over a period of two years.
The German government and the White House's National Security Council spokeswoman both declined to comment following Friday's reports.
ksb/sb (dpa, Reuters)