This is the 17634th most frequent Russian word.
убийствами
"Убийствами" means "murders" (instrumental plural form).
In this sentence, 'убийствами' is the instrumental plural form of 'убийство', meaning 'murders'. It indicates the means by which the investigation is focused.
Полиция занимается расследованием убийствами, чтобы найти виновных.
The police are investigating murders to find the culprits.
Here, 'убийствами' again is the instrumental plural form, signifying the association or handling of topics related to murders.
Историки изучают информацию, связанную с убийствами в средневековье.
Historians study the information related to murders in the Middle Ages.
In this usage, 'убийствами' portrays the concept of murders as an aspect of the story plot, in instrumental case emphasizing the relation.