This is the 12346th most frequent Polish word.
morderstw
The Polish word "morderstw" means "murders" (genitive plural form of "murder").
In this sentence, 'morderstw' is used as a possessive plural noun referring to acts of murder, indicating a group or category of criminal acts.
To było jedno z najbardziej tajemniczych morderstw w historii miasta.
It was one of the most mysterious murders in the history of the city.
Here, 'morderstw' is again a possessive plural noun, indicating specific acts that are the objective of research or analysis.
Od lat badano sposób popełniania tych morderstw.
For years, the manner of committing these murders has been researched.
In this case, 'morderstw' is used in a quantitative context, denoting the plural form and indicating a count of occurrences.