grammaticality - "They interviewed several candidates who/whom he thought had the experience he required."

They interviewed several candidates who he thought had the experience and qualifications he required.

My test prep book says this should be "who" because of the subordinate clause's predicate:

They interviewed several candidates who he thought had the experience and qualifications he required.

I feel like it should be "whom" as it's the object of the main clause.

The interviewed several candidates whom he thought had the experience and qualifications he required.

Who's right?