BOSTON—Extremist U.S. newspapers and other publications found in the apartment of Boston Marathon bombing suspect Tamerlan Tsarnaev reveal a broad interest in far-flung conspiracy theories, well beyond the Islamist radicalism authorities allege motivated the attack.

Mr. Tsarnaev discovered some of the radical publications by chance. He had worked caring for a 67-year-old man who passed on the newspapers and his fringe beliefs long before Mr. Tsarnaev and his brother allegedly set off explosives that killed three people and...