The report found President Vladimir Putin personally directed the Russian efforts to hack computer networks and accounts affiliated with the Democratic Party and leak information damaging to Clinton.
The panel,formally called the Senate Select Committee on Intelligence,also alleged Manafort collaborated with Russians,including oligarch Oleg Deripaska and an alleged Russian intelligence operative,Konstantin Kilimnik,before during and after the election.
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The panel found Manafort's role and proximity to Trump created opportunities for Russian intelligence,saying his"high-level access and willingness to share information with individuals closely affiliated with the Russian intelligence services ... represented a grave counter-intelligence threat".
It was not clear what effect,if any,the report might have on the current US presidential campaign in which Trump faces Democrat Joe Biden in the November 3 US election.
Opinion polls show former vice-president Biden has built an expansive lead in nearly every battleground state that Trump won narrowly in 2016,as the Republican's approval numbers tumble amid the coronavirus pandemic.
Russia's alleged election interference,which Moscow denies,sparked a two-year-long US investigation headed by Special Counsel Robert Mueller.