The FBI on Tuesday released more than 400 pages of its files on left-wing activist Saul Alinsky that showed the bureau, for years, monitored the community organizer and investigated accusations that he was a communist.

Alinsky, who authored the book “Rules for Radicals” and was known for his activism in Chicago, died in 1978. A self-described “radical,” Alinsky never identified as a communist. But the FBI, for years, harbored suspicions about his sympathies.

The FBI files show the bureau received a tip in 1940 questioning if Alinsky was a communist. The FBI in Chicago wrote in one document that the bureau was interested in obtaining information about whether Alisnky was involved in organizing a “communistic or subversive group.”

A 1941 FBI document, though, said the Chicago Police Department conducted an investigation and found “no information that he is communistically inclined.”

The police investigation, the document said, “did not disclose that Alinsky is a member of any un-American organization nor did it disclose that he had ever made any remarks or exhibited any acts against the United States government, or in favor of any foreign government.”

Still, the files indicate the bureau continued to probe for years after that to see whether Alinsky had communist sympathies. It included newspaper clippings about Alinsky’s work organizing in black communities, as well as memos of findings from “confidential sources” who provided the bureau information on Alinsky’s activities.

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A 1965 letter to then-FBI Director J. Edgar Hoover from a person whose name was redacted included news clippings about Alinsky and said, “The situation that amazes me about this enclosed clipping is the fact that our Constitution permits people freedom—people who try to tear it down.”

“I wish I could, personally, do something about a man like this,” the person wrote in the letter to Hoover.

But the sender said, “all I can do is to be on the alert, respect law and order and justice and be a loyal American to our Democracy.”

In recent years, Republicans have resurrected Alinsky’s name to use as a weapon against Democrats in presidential elections: 2016 Democratic presidential candidate Hillary Clinton met with Alinsky in 1968 while writing her college thesis on his community organizing and President Obama, like Alinsky, famously worked as a community organizer in Chicago before getting into politics.