“At that point it was clear that lists of targeted foreign officials may well become public,” said one official, “so many of the interceptions were suspended.”

The N.S.A.’s documentation on Ms. Merkel’s case authorized the agency’s operatives in Germany not only to collect data about the numbers she was calling, but also to listen in on her conversations, according to current and former administration officials.

It was unclear whether excerpts from Ms. Merkel’s conversations appeared in intelligence reports that were circulated in Washington or shared with the White House. Officials said they had never seen information attributed to an intercept of Ms. Merkel’s conversations. But they said it was likely that some conversations had been recorded simply because the N.S.A. had focused on her for so long.

In both public comments and private interchanges with German officials, the Obama administration has refused to confirm that Ms. Merkel’s phone was targeted, though it has said that it is not the subject of N.S.A. action now, and will not be in the future.

The refusal to talk about the past has further angered German officials, who have said the surveillance has broken trust between two close allies. The Germans were particularly angry that the operation appears to have been run from inside the American Embassy or somewhere near it, in the heart of Berlin, steps from the Brandenburg Gate.

None of the officials and former officials who were interviewed would speak directly about the decision to target Ms. Merkel, saying that information was classified. But they said the legal distinction between tapping a conversation and simply collecting telephone “metadata” — essentially the kind of information about a telephone call that would be found on a telephone bill — existed only for domestic telephone calls, or calls involving United States citizens.

To record the conversation of a “U.S. Person,” the intelligence agencies would need a warrant. But no such distinction applies to intercepting the calls of foreigners, on foreign soil — though those intercepts may be a violation of local law.