Cedar Rapids, Iowa (CNN) Sen. Elizabeth Warren tweeted on Friday evening that "Congress is complicit" in failing to start impeachment proceedings against President Donald Trump after the news broke that Trump had pressed Ukraine's President to investigate Democratic presidential candidate Joe Biden's son, according to a person familiar with the situation.

Asked by CNN at Saturday's Iowa Steak Fry as she was leaving the grill area what steps Congress needed to take, the Massachusetts senator said, "Impeachment, we need to start impeachment proceedings."

Since then, she has repeatedly called for the House to begin impeachment proceedings but took it a step further Friday by calling Congress "complicit" in "failing to act."

"After the Mueller report, Congress had a duty to begin impeachment," she wrote in a tweet Friday evening . "By failing to act, Congress is complicit in Trump's latest attempt to solicit foreign interference to aid him in US elections. Do your constitutional duty and impeach the president."

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