WASHINGTON (AP) — Former CIA Director John Brennan told Congress Tuesday he personally warned Russia last summer against interfering in the U.S. presidential election and was so concerned about Russian contacts with people involved in the Trump campaign that he convened top counterintelligence officials to focus on it.

Brennan's testimony to the House intelligence committee was the clearest public description yet of the significance these contacts play in counterintelligence investigations that continue to hang over the White House.

Brennan, who was President Barack Obama's CIA director, said he couldn't say whether there was collusion between Russia and the Trump campaign, an issue being investigated by the FBI and congressional committees.

"I don't have sufficient information to make a determination about whether or not such cooperation or complicity or collusion was taking place," Brennan said. "But I know there was a basis to have individuals pull those threads."

President Donald Trump has predicted the investigations won't find collusion, and his efforts to cast doubt and curb the probes have led to the appointment of a special counsel at the Justice Department.

News reports that Trump asked his national intelligence director and National Security Agency chief to state publicly there was no evidence of collusion have heightened criticism.

Dan Coats, the current U.S. director of national intelligence, declined to comment Tuesday on a Washington Post report that said the president had asked him to publicly deny any collusion between Russia and Trump's campaign.

Coats told senators at a separate hearing that it would be inappropriate to discuss private conversations with the president.

Nevertheless, Mark Warner of Virginia, the top Democrat on the Senate intelligence committee, said Coats and NSA director Mike Rogers should provide explanations.

The White House said the hearings support the administration's version of events.

A day earlier, Trump's first national security adviser, Michael Flynn, invoked his constitutional right not to incriminate himself in response to the Senate committee's request for details about interactions between him and the Russians. Trump associates Paul Manafort and Roger Stone have provided the committee with information, while former campaign adviser Carter Page has not.

"I can only say I have fully complied with their specific request," Stone told The Associated Press in an email Tuesday. He said he told the committee he remains ready to testify without immunity and in public.

The Senate panel on Tuesday decided to issue two additional subpoenas to Flynn's businesses and sent a letter to his lawyer asking about the legal basis for his invoking his Fifth Amendment right over documents as opposed to testimony. The chair of that committee, Richard Burr of North Carolina, said if there is no response from Flynn, the committee may consider pursuing a contempt of Congress charge.

"Everything is on the table," Burr said. Everything but immunity, he added.

Investigators also have questions about contacts between the Russians and Trump's son-in-law Jared Kushner and Attorney General Jeff Sessions.

Brennan said he had seen intelligence that "revealed contacts and interactions" between Russian officials and Americans "involved" in the Trump campaign. He said this was cause for concern "because of known Russian efforts to suborn such individuals, and it raised questions in my mind, again, whether or not the Russians were able to gain the cooperation of those individuals."

In late July of last year there was so much concern that he convened a group of officials from the CIA, FBI and National Security Agency to focus on it exclusively.

"The purpose was to ensure that experts in key agencies had access to information and intelligence relevant to Russian actions so that we could have as full an appreciation as possible on the scope, nature, and intentions of this Russian activity," Brennan said.

He said he personally warned the Russians in August to stop interfering in the U.S. democratic process, telling a senior Russian security official that continued meddling would backfire and prevent any warming of relations after the election.

He said the Russian official denied such interference but also said he would relay the concern to President Vladimir Putin.

Trump, currently on a nine-day international trip, has had his own conversations with the Russians questioned in light of reports that he shared extremely classified intelligence with Russian diplomats in the Oval Office on May 10.

Brennan said that while he was CIA director he shared classified information with Russia and other nations about threats related to terrorism. But if reports about what Trump shared with the Russians are true, he said, it would be a violation of protocol. This type of information is typically shared in intelligence channels and not between the U.S. president and foreign diplomats, Brennan said.

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Associated Press writers Eric Tucker and Jeff Horwitz contributed to this report.