BERLIN — In the summer of 2011, American intelligence agencies spied on a senior German official who they concluded had been the likely source of classified information being leaked to the news media.

The Obama administration authorized the top American spy in Germany to reveal to the German government the identity of the official, according to German officials and news media reports. The decision was made despite the risk of exposing that the United States was monitoring senior national security aides to Chancellor Angela Merkel.

The tip-off appears to have led to a senior German intelligence official being barred from access to sensitive material. But it also raises suspicions that Ms. Merkel’s government had strong indications of the extent of American surveillance at least two years before the disclosures by Edward J. Snowden, which included the number of a cellphone used by the chancellor.

The decision by the United States to risk disclosing a surveillance operation against a close ally indicates the high level of concern over the perceived security breach. It is unclear, however, what that information might have been or if it involved intelligence provided to Germany by the United States.