This is the 6238th most frequent Russian word.
убийцу
"Killer" or "murderer."
In this sentence, 'убийцу' is in the accusative case because it is the direct object of the verb 'поймала' (caught).
Полиция поймала убийцу.
The police caught the killer.
Here, 'убийцу' is also in the accusative case, as it is the object of the verb 'опасаюсь' (I fear).
Я опасаюсь убийцу, который всё ещё на свободе.
I fear the killer who is still at large.
In this context, 'убийцу' appears in the accusative case due to the preposition 'об' (about) and the noun 'историю' being its direct object.