WASHINGTON — The C.I.A. told senior lawmakers in classified briefings last summer that it had information indicating that Russia was working to help elect Donald J. Trump president, a finding that did not emerge publicly until after Mr. Trump’s victory months later, former government officials say.

The briefings indicate that intelligence officials had evidence of Russia’s intentions to help Mr. Trump much earlier in the presidential campaign than previously thought. The briefings also reveal a critical split last summer between the C.I.A. and counterparts at the F.B.I., where a number of senior officials continued to believe through last fall that Russia’s cyberattacks were aimed primarily at disrupting America’s political system, and not at getting Mr. Trump elected, according to interviews.

The former officials said that in late August — 10 weeks before the election — John O. Brennan, then the C.I.A. director, was so concerned about increasing evidence of Russia’s election meddling that he began a series of urgent, individual briefings for eight top members of Congress, some of them on secure phone lines while they were on their summer break.

It is unclear what new intelligence might have prompted the classified briefings. But with concerns growing both internally and publicly at the time about a significant Russian breach of the Democratic National Committee, the C.I.A. began seeing signs of possible connections to the Trump campaign, the officials said. By the campaign’s final weeks, Congress and the intelligence agencies were racing to understand the scope of the Russia threat.