“I encountered and am aware of information and intelligence that revealed contacts and interactions between Russian officials and U.S. persons involved in the Trump campaign that I was concerned about because of known Russian efforts to suborn such individuals,” Mr. Brennan told lawmakers on the House Intelligence Committee.

“It raised questions in my mind about whether Russia was able to gain the cooperation of those individuals,” he said, adding that he did not know whether the Russian efforts were successful.

He added, “I don’t know whether such collusion existed.”

It was the first time he publicly acknowledged that he was concerned about possible ties between Russia and the Trump campaign.

He said he left office in January with many unanswered questions about the Russian influence operation. Intelligence officials have said Russia tried to tip the election toward Mr. Trump.

Mr. Brennan became so concerned last summer about signs of Russian election meddling that he held urgent, classified briefings for eight senior members of Congress, speaking with some of them over secure phone lines while they were on recess. In those conversations, he told lawmakers there was evidence that Russia was working to elect Mr. Trump president.