Authorities are investigating a "suspicious" letter after it was sent to the home of Sen. Susan Collins, R-Maine, Monday afternoon.

Police, a fire crew, and a HAZMAT team were among the first to respond to the report of the letter mailed to Collin's property in Bangor, Maine, according to CNN.

Bangor Police Department Sgt. Wade Betters told reporters at a press conference Monday that members of his Criminal Investigation Division would be spearheading the probe.

[Also read: Alumni, faculty push to revoke Susan Collins' honorary degree over Kavanaugh vote]

"We're the primary responding agency, so that kind of falls to us and our jurisdiction. But then we turn the case over to the U.S. Capitol Police, potentially federal investigators to see it through," Betters said.

Betters did not provide details regarding the contents of the letter, nor what about it sparked the initial concern. The note comes after Collins became a key vote in Supreme Court Justice Brett Kavanaugh's confirmation process, which prompted threats before his nomination was considered on the Senate floor.

Collins' office did not immediately respond to the Washington Examiner's request for comment. A spokesperson told Fox News that the senator was “on her way home" and only her husband was at the house when the letter was delivered.