Is the sentence
The police is looking for the culprit who is suspected to be the mastermind
A simple,compound or complex sentence. I think it is a simple sentence
Is the sentence
The police is looking for the culprit who is suspected to be the mastermind
A simple,compound or complex sentence. I think it is a simple sentence
This is a complex sentence because it contains one independent clause and one relative clause.
Independent clause
The police are looking for the culprit
Relative clause
who is suspected to be the mastermind
There is an error in the subject-verb agreement of the sentence. 'Police' is one of those tricky collective nouns that always treats the group as individuals and is considered plural even when discussing them as a group.
This is a complex sentence.
It consists of the main clause (The police is looking for the culprit)
and the defining relative clause
(who is suspected to be the mastermind).