As the dust settles on Russian interference in the United States election, journalists are confronting an aspect that has received less scrutiny than the hacking itself but poses its own thorny questions: Moscow’s ability to steer Western media coverage by doling out hacked documents.

Reporters have always relied on sources who provide critical information for self-interested reasons. The duty, tricky but familiar, is to publicize information that serves the public interest without falling prey to the source’s agenda.

But in this case, the source was Russia’s military intelligence agency, the G.R.U. — operating through shadowy fronts who worked to mask that fact — and its agenda was to undermine the American presidential election.

By releasing documents that would tarnish Hillary Clinton and other American political figures, but whose news value compelled coverage, Moscow exploited the very openness that is the basis of a free press. Its tactics have evolved with each such operation, some of which are still unfolding.