A hazardous materials team was called to the Bangor, Maine home of Sen. Susan Collins Monday after a threatening letter containing what its author said was the poison ricin was delivered there.

Collins spokesperson Annie Clark said on Twitter that Collins' husband, Tom Daffron, the couple's dog and parts of the home were quarantined while the letter was analyzed. Clark added that the home was cleared and the couple "will be able to remain at home tonight."

Outside the house, a Bangor Daily News photographer captured an image of a person in a hazmat suit holding an envelope in a plastic bag. Later, the FBI arrived, along with vans carrying people in military uniforms, the newspaper reported.

Law enforcement officials were analyzing the contents of the letter and the investigation into its origins was ongoing Monday night. An FBI spokeswoman told The Associated Press that preliminary tests on the envelope indicated there was no threat to the public.

It was unclear who sent the letter and why. But critics have hurled threats at Collins recently over her vote to confirm Brett Kavanaugh to the Supreme Court amid historic allegations of sexual misconduct against the federal judge.

Clark tweeted late Monday night that the envelope incident was "the latest in a series of threats against Senator Collins, her loved ones, and her staff."

Independent Sen. Angus King of Maine, who voted against Kavanaugh, came to his fellow senator's defense.

"Regardless of any political differences, @SenatorCollins, her family, and her staff should not have to be subjected to these threats — there's just no place for it in our discourse," he tweeted.

In a statement, Collins and Daffron said they were "very grateful for the immediate and professional assistance that we received" from the authorities.

"We are also truly appreciative of the many well wishes that we received today," the couple added. "Our friends and neighbors have been incredibly kind and have even offered to open their homes to us. We feel blessed to live in such a supportive community."

Collins signaled that she would vote for Kavanaugh in a dramatic Senate floor speech on Sept. 28. The judge was confirmed the following day by a 50-48 vote.

But in the wake of Collins’ support, activists lashed out against her and her fellow Senate colleagues who voted to confirm Kavanaugh.

“Senator Susan Collins is the mother & grandmother of white women in America who gave us a Donald Trump presidency. The 53%,” tweeted Linda Sarsour, a far-left activist and a leader of the Women’s March that branded Collins a “rape apologist” after the speech. “She is a disgrace & her legacy will be that she was a traitor to women and marginalized communities. History will not treat her kindly.”

“A white woman Senator is talking about presumption of innocence that is never offered to Black men in America. You are watching white supremacy live on the Senate Floor,” Sarsour said even before Collins finished her speech.

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Kat Calvin, the founder of anti-voter ID group “Spread the Vote” that claims to be non-partisan, urges people to publicly harass Collins over her voting decision.

“Never let Collins have a moment of peace in public again,” she wrote.

An anti-Collins fundraiser page raised nearly $3 million in pledges by the eve of Kavanaugh’s confirmation vote, with the money going to her future opponent.

“If you fail to stand up for the people of Maine and for Americans across the country, every dollar donated to this campaign will go to your eventual Democratic opponent in 2020,” the fund’s organizers wrote. “We will get you out of office.”

Fox News’ Lukas Mikelionis and The Associated Press contributed to this report.