Qui est-ce can mean both -- who and whom.

Tu clarify: qui is the personal interrogatory pronoun for a person, que is the same for inanimate objects.

Qui as-tu rencontré -- Whom did you meet. (qui stands for a person and is the direct object of rencontrer)
Qui t'a salué? -- Who greeted you. (qui is the subject of the sentence)

Attention: don't confuse qui/que as interrogatory pronouns with qui/que as relative pronouns. Here, qui is for the subject and que for a direct object.

Qui est-ce que tu as rencontré. -- Who is it, whom you met. (Qui is the interrogative pronoun, animate object and subject of the main clause and que the relative pronouns and direct object of the subjunctive clause.)

Qu'est-ce qui s'est passé? -- What is it that happened? (Que is the interrogative pronoun, inanimate object and subject of the main clause, qui is the relative pronoun and subject of the relative clause).