A term (two words combined) of "questioned cost" is a terminology used in Accouting/audit world. It has a specific meaning.

My question determining the part of speech of each words, for example, what is the part of speech of 1) questioned 2) cost

What I thought it should be is not after examining each words (see below).

1) "questioned" is not an adjective. That word comes from a word "question" which is a noun or verb. So, i am confused because I thought the "questioned" was an adjective modifying a word "costs"(noun).

What is the part of speech for "questioned"? and how it is able to modify the word "cost" if it is not an adjective.

Thank you for your advice~!!

Actually, it is an adjective. More specifically, it's a participle (which is an adjective formed from a verb.)