Here's why:
* Prepositional phrase: It's a combination of a preposition ("by") and its object ("passed").
* Adverb: It tells us how, when, or where the verb happened.
Example:
The ball passed by the window.
* "Passed" is the verb.
* "Passed by" tells us how the ball moved (the manner of the action).
Therefore, "passed by" acts as an adverbial phrase in this sentence.