A series of meetings between Russians and three Trump campaign advisers in a six-week span last year provide the strongest evidence yet that the Kremlin had mounted an intelligence operation aimed at infiltrating the campaign, according to former intelligence officials and experts.

The recent acknowledgments by advisers Carter Page and George Papadopoulos that they talked with Russians and Russian officials around the same time that Donald Trump Jr. met with a Kremlin advocate indicates a coordinated effort to recruit Trump associates, possibly for eliciting information and conveying messages, intelligence experts told BuzzFeed News.

“I don’t think there can be any doubt now that the Russians were targeting the Trump campaign, and there were members of the Trump campaign who were receptive to those approaches,” said Asha Rangappa, a former FBI counterintelligence agent in New York who now lectures at Yale University’s Jackson Institute for Global Affairs.

The failure of both Trump Jr. and Papadopoulos to inform US authorities that their Russian contacts said they had incriminating information about Hillary Clinton would have emboldened Russian intelligence to continue reaching out to Trump associates, the experts told BuzzFeed News.

“Their overtures weren't rejected, and there was presumably no reporting of the [Trump Jr.] meeting by the Trump side to the appropriate authorities. They would have felt confident they weren't dealing with a reluctant partner,” said Rolf Mowatt-Larssen, who was a CIA intelligence officer for 23 years and director of intelligence and counterintelligence at the US Energy Department.

The contacts illustrate the overlooked role in the 2016 election of Russian intelligence, which has flourished under Putin, a former spy. While President Donald Trump continues to say he's skeptical that Russia interfered in the election and denies that there was any collusion between his campaign and Russian authorities, outside experts say they believe there can now be little doubt that the Russian government was trying to penetrate the campaign.

The timing of Russia’s outreach was critical to the way the probe into Russia’s election meddling has unfolded.

Foreign policy advisers Page and Papadopoulos revealed in recent congressional testimony and court documents that between April 2016 and July 2016 they met, spoke, and exchanged emails with Russians who gushed about Trump and wanted to bring him closer to Russian President Vladimir Putin.

In May 2016, as Trump sealed the Republican presidential nomination and expressed optimism about relations with Putin, he installed as campaign chairman Paul Manafort, a Republican operative who consulted for a Russia-friendly Ukrainian political party from 2006 to 2015.

In June 2016, in the so-called Trump Tower meeting, Trump Jr. met a Kremlin-connected Russian lawyer who he’d been told wanted to share incriminating information about Hillary Clinton. Manafort and Trump's son-in-law Jared Kushner also attended the meeting.

The FBI launched a counterintelligence investigation one month later, reacting to what former CIA director John Brennan told Congress was growing alarm over “contacts and interactions between Russian officials and US persons involved in the Trump campaign.” The investigation became special counsel Robert Mueller’s probe into possible coordination between the Trump campaign and Russia’s meddling in the 2016 presidential election.

The Russian outreach to Page, Papadopoulos, and Trump Jr. brims with hallmarks of Russian intelligence, including the use of nongovernment intermediaries, or “cutouts,” whose unofficial status would let the Kremlin deny involvement, the experts said. It also shows the ambition of Russian intelligence, which has been overshadowed by reports of its social-media efforts to influence the 2016 election.

“This just smacks of a classic intelligence operation,” John Sipher, a retired CIA officer who lived in Russia in the 1990s and was deputy of the CIA’s Russia program in the early 2000s, told BuzzFeed News.

US officials have not said what contacts triggered the investigation into Russian intelligence efforts. On Sunday, former national intelligence director James Clapper Jr. told CNN’s State of the Union that he had he not known while in office that Trump Jr. agreed to meet with a Russian lawyer promising dirt on Clinton or that a professor had told Papadopoulos that “the Russians” had damaging information on her as well as emails.

If Page, Papadopoulos, or Trump Jr. detected anything suspicious about the motives of their Russian contacts, there’s no record of it. Brennan noted in congressional testimony in May that “frequently, people who go along a treasonous path do not know they are on a treasonous path until it’s too late.”

Trump, who has questioned the US intelligence community’s assessment that Russia interfered in the election, said Saturday that he believed Putin’s statement to him at a summit in Vietnam that Russia did not meddle.