From "Leaking: Who Does It? Who Benefits? At What Cost?" -- a paper written by Elie Abel for the Twentieth Century Fund:

It is difficult to conceive of a constitutional remedy that would end the practice of leaking without doing violence to the fabric of American freedoms. There is room, however, for voluntary reforms. For example, a decent skepticism about the motives of leakers on the part of the media would help. The leaker's interest or bias should be identified, even when he cannot be named. . . .

There are relatively few examples, unfortunately, of reporters who have taken the trouble to identify the source's bias in this fashion. One notable exception was a story by Robert Pear in The New York Times, on April 25, 1987, that ran under the headline:

"DUARTE APPEALS TO REAGAN TO LET SALVADORANS STAY."

The Salvadoran president, Jose Napoleon Duarte, had sent a confidential letter to Mr. Reagan expressing alarm over the effects of the new U.S. immigration law on "some 400,000 to 600,000 Salvadorans" who had entered the country illegally since 1982 and would, therefore, not qualify for amnesty under the terms of the new legislation. Duarte requested temporary refuge for them, contending that their deportation would be disastrous to the Salvadoran economy.

Pear dealt with the attribution problem in a single short paragraph: "A copy of Mr. Duarte's letter was obtained by The New York Times from a person sympathetic to his request and to the plight of Salvadorans in the United States." That one sentence told the reader all he needed to know about the motives of the person who leaked the letter to the Times.