The bombshell revelation that President Donald Trump's oldest son Don Jr. met with a Kremlin-tied Russian lawyer hawking damaging material on Hillary Clinton has taken suspicions of election collusion with Moscow to a new level.
There are several probes into whether the Trump team colluded with Russians to swing the election away from Clinton, but there is so far no conclusive proof.
The increased evidence of contacts between the two sides however gives weight to the ominous investigation by Justice Department independent prosecutor Robert Mueller.
Here are the key figures in the probe:
So far, there is nothing directly tying the president to Russian interference, and he has vehemently denied any involvement.
But the actions of his top aides, and his refusal to strongly condemn Russian interference, have fueled lingering suspicions.
Trump is facing allegations that he attempted to obstruct the investigation by firing FBI director James Comey in May and asking members of Congress and the heads of his intelligence agencies to intervene on his behalf.
Last June, Donald Jr -- who runs the family real estate business -- met with Kremlin-linked lawyer Natalia Veselnitskaya, together with campaign chairman Paul Manafort and the president's son-in-law Jared Kushner.
Veselnitskaya apparently pitched the meeting as an offer of material that could incriminate Clinton, sourced from the Russian government.
The material "is part of Russia and its government's support for Mr. Trump," Trump Jr. was told in an email by an intermediary.
"If it's what you say I love it," he emailed back.
But when they finally met, Trump Jr. says Veselnitskaya had "no meaningful" information on Clinton and instead wanted to talk about US sanctions on Russia. He labelled it a "nonsense meeting."
Trump's one-time national security advisor and the former US military intelligence chief was named by Republican activist Peter W. Smith as his main link to the campaign as he sought to obtain damaging Clinton emails from Russian hackers.
But Flynn has other troubles: the retired general accepted money from Russian companies for speeches and appearances before the election campaign, possibly violating rules for former military officers on dealings with US adversaries.
And he repeatedly communicated with Russian ambassador Sergey Kislyak before and after the election. His alleged lying about those contacts to investigators puts him at risk of perjury charges.
The husband of Trump's daughter Ivanka attended the meeting with the Russian lawyer allegedly offering damaging Clinton materials.
He and Flynn met Kislyak in December, where he reportedly offered to set up a secret communications channel with Moscow using Russian facilities, according to The Washington Post.
Later that month, Kushner also met with Sergey Gorkov, chief of Vnesheconombank and a former member of Russian intelligence. The bank, a key arm of the Russian government, is under tough US sanctions. The topic if their meeting is so far unknown.
Manafort managed Trump's campaign until he was forced to resign in August amid reports that he was under investigation over millions of dollars he was allegedly paid by the pro-Russia former Ukrainian president Viktor Yanukovych.
Manafort's private consulting business on behalf of Moscow-linked billionaire oligarchs have fueled concerns about possible collusion. He also took part in Trump Jr.'s meeting last year with the Russian lawyer who had promised to provide damaging material on Clinton.
The former political dirty tricks master for president Richard Nixon has been open about pulling out all the stops to get Trump elected.
He communicated during the campaign with WikiLeaks founder Julian Assange and hacker "Guccifer 2"; both released communications stolen from Clinton and her campaign.
US intelligence links Guccifer 2 to Russian intelligence, and says WikiLeaks acted, perhaps unknowingly, as a front for Russia leaking the damaging Democratic Party materials.