Now we know why Angela Merkel was so upset yesterday: According to a report published by the Guardian, the NSA monitored the phone calls of at least 35 world leaders, using phone numbers provided by a U.S. official in another government department.

The report is based on an Edward Snowden-provided 2006 classified memo, titled “Customers Can Help SID Obtain Targetable Phone Numbers,” which sought the help of U.S. officials and provided examples of previous successful contributions. From the Guardian:

"In one recent case," the memo notes, "a US official provided NSA with 200 phone numbers to 35 world leaders … Despite the fact that the majority is probably available via open source, the PCs [intelligence production centers] have noted 43 previously unknown phone numbers. These numbers plus several others have been tasked."

The document continues by saying the new phone numbers had helped the agency discover still more new contact details to add to their monitoring: "These numbers have provided lead information to other numbers that have subsequently been tasked."

The eavesdropping was routine, though it produced "little reportable intelligence,” according to the Bush-era memo.

Yesterday, German Chancellor Merkel called Obama to confront him about allegations that the NSA monitored her cell phone, reportedly after she was tipped off by a Der Spiegel investigation. Obama assured Merkel that the U.S "is not monitoring and will not monitor" her phone calls, though reportedly stopped short of saying such monitoring had never occurred in the past.

[Image via AP]