Learn Finnish: Kumpikaan ei liene kiristänyt murhaajaa. - Neither probably blackmailed the murderer.

This is the negative perfect form of the potential mood. It is formed with the negative verb ei, the potential connegative of the verb to be (liene), and the active past participle of the main verb (kiristänyt). Together, these three words express an assumption about a past event that likely did not happen.

I know the verb kiristää as "to tighten". Why does it mean "blackmailed" here?

While kiristää literally means "to tighten" (like a knot or a screw), it is used figuratively in Finnish to mean "to extort" or "to blackmail". The context of the word murhaajaa makes it clear that we are talking about extortion rather than physical tightening.

Murhaajaa is in the partitive case. Is this simply because it is a negative sentence?

The negative verb ei does require the object to be in the partitive case, so that rule definitely applies here. However, kiristää in the sense of blackmailing is inherently a partitive verb, meaning it would take a partitive object even in an affirmative sentence.

Why is the pronoun kumpikaan used here instead of kumpikin?