Rochester, New York - A man is suing after he was threatened by federal officers over an email he sent criticizing then-Acting US Immigration and Customs Enforcement Director Todd Lyons over the shooting of Alex Pretti in January.

Five months after he sent an email rebuking Lyons over Pretti's killing, David Streever received a knock at the door and a visit from two ICE officers who warned him the message could have been perceived as a threat.

To Streever, the June 23 visit was a clear breach of his First Amendment right to freedom of speech. The Rochester man on Monday filed a lawsuit against the Department of Homeland Security alleging an attempt to "confront and intimidate him."

"The First Amendment unquestionably protects Streever's criticism," the Foundation for Individual Rights and Expression (FIRE) – a nonprofit civil rights group representing Streever – said in a statement.

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"Writing an angry note to political leaders is an American tradition as old as the republic itself. That's why time and again, the Supreme Court has upheld that cherished freedom, warning against attempts to label heated political rhetoric as dangerous or unprotected."

In the email itself, Streever called Lyons "a monstrous human being" and compared him to Reinhard Heydrich, the high-ranking SS Officer who is seen as one of the chief architects of the Holocaust.

"The way you are protecting the obvious execution in Minnesota, even as we see the videos, will lead to your downfall," he wrote. "You will never know peace… You will torment yourself until your last day on Earth."

In a statement cited by CNN, a DHS spokesperson said ICE "investigates all credible threats towards its employees and officers," but clarified that "we do not comment on any ongoing investigations."