Learn Portuguese (Portugal): A polícia procura o assassino. - The police are looking for the murderer.

In English, "police" is usually treated as a plural noun, but in Portuguese, a polícia is a singular feminine noun referring to the police force as a single collective unit. Therefore, the verb must be in the third-person singular: procura.

Why isn't there a preposition like por after procura to mean "looking for"?

The verb procurar inherently means "to look for" or "to search for". Because the "for" is already built into the meaning of the verb, you don't need to add an extra preposition. The object of the search (o assassino) simply follows the verb directly.

The English translation uses the present continuous "are looking". Could I use está a procurar here instead?

Yes, you could. A polícia está a procurar focuses on the action happening right at this very moment. However, the simple present procura is widely used in Portuguese for ongoing, extended situations (like an active police manhunt), so it sounds very natural in this context.

Does assassino mean a paid "assassin", or any "murderer"?

It covers both! While English usually distinguishes between a general murderer and a politically motivated or paid assassin, Portuguese uses assassino for anyone who commits a murder.