AMHERST - Before the Senate or the House initiate impeachment proceedings against President Donald J. Trump, there needs to be a full, independent investigation of him and his relationship with Russia, U.S. Sen. Elizabeth Warren said Friday.

A special prosecutor should be named to examine the relationship between "Trump, the Trump campaign and a hostile foreign government," Warren said in a brief interview before her University of Massachusetts graduation address.

"This is why it's so significant that Donald Trump fired the head of the FBI," she said. "The idea of trying to stop an independent investigation that we can't get to the bottom of it, that's the part of it that's a real threat."

Warren was answering a question about the call to impeach President Trump and the firing of FBI Director James Comey, who was in the middle of an investigation into possible collusion between the Trump campaign and Russia, something Trump has called a "witch hunt."

Before Comey's firing, Warren said, the most insistent issue was health care bill. Legislation that passed the House recently wasn't a health care bill, she said, it was a tax cut bill.

Warren argued the bill will raise costs for middle class families while producing a "tax cut for a handful of millionaires and billionaires."

The senator said she remains committed to the work she's doing and to people in this state and has no plans to run for president herself. She also said she loves giving graduation speeches and that she wrote the speech she was about to give herself.