The undercover agent still investigating the case.

run-on sentence

fragment

complete

I believe this would be a complete sentence.

However, perhaps you meant to put "The undercover agent is still investigating the case."

Hope this helps!

from the excerpt from the "the dark game" the reader can infer that folke cronholm was spying for the allied nations.

it creates a visual picture of the man.

it conveys what the scene sounds like.

it describes the actions of the boy.

it reveals the poet’s opinion of the characters.

it encourages readers to care for the characters.