The word "conspiracy" does not imply lack of evidence (although it's often used that way in popular culture). In fact, "conspiracy" is a legal term [1] describing an illegal act of collusion.

And in this case, part of the conspiracy was confirmed by the court, and there were some convictions. From Wikipedia:

    In 1949, Firestone Tire, Standard Oil of California,
    Phillips Petroleum, GM and Mack Trucks were convicted
    of conspiring to monopolize the sale of buses and
    related products to local transit companies
    controlled by NCL and other companies ...

[1] https://en.wikipedia.org/wiki/Conspiracy_(civil), https://en.wikipedia.org/wiki/Conspiracy_(criminal)