La policía investigó la causa.

Because la policía here refers to the police as an institution/force, and that noun is treated as feminine singular in Spanish.

By contrast:

• el policía = a male police officer
• la policía = a female police officer, or the police force, depending on context

So in this sentence, la policía is most naturally understood as the police.

It can mean either, depending on context.

In real use:

• la policía = the police very often
• la policía = the policewoman in some contexts

If a speaker wants to avoid ambiguity, they might say:

• la agente = the female officer
• los policías = the police officers
• el cuerpo de policía = the police force

So the sentence could technically be ambiguous by itself, but context usually makes the meaning clear.

Because Spanish treats la policía as a singular collective noun when it means the police force.

So you get:

• La policía investigó... = singular verb

But if you mean individual officers, you would use a plural noun and plural verb:

• Los policías investigaron...

This is different from English, where the police usually takes a plural verb.

Investigó is the preterite tense, third person singular, from investigar.

So:

• investigar = to investigate
• investigó = he/she/it investigated

The preterite is used for a completed action in the past.

Here it presents the investigation as a finished event:

• La policía investigó la causa. = the police investigated the cause.

A different tense would change the nuance:

• investigaba = was investigating / used to investigate
• ha investigado = has investigated

The accent mark shows that the stress falls on the last syllable: in-ves-ti-GÓ.

It also helps distinguish it from investigo:

• investigo = I investigate
• investigó = he/she/it investigated

This is very common with many -ar verbs in the preterite:

• habló
• compró
• investigó

The accent mark in policía shows that the stress is on -cí- and that í-a is pronounced as two syllables, not one.

So it is pronounced roughly as:

• po-li-CÍ-a

That gives the word four syllables:

• po
• li
• cí
• a

Without the accent, Spanish pronunciation rules would suggest a different stress pattern.

Spanish uses articles like el, la, los, las more often than English does.

Here, la causa refers to a specific cause, so the definite article is natural:

• la causa = the cause

Spanish usually does not drop the article in a sentence like this when talking about a specific noun.

Causa usually means cause, reason, or sometimes a legal cause/matter, depending on context.

But if you mean a police case, the more usual Spanish word is:

• caso = case

So:

• La policía investigó la causa = The police investigated the cause.
• La policía investigó el caso = The police investigated the case.

For an English speaker, this is an important distinction because cause and case are not interchangeable in Spanish.

Because la causa is a thing, not a person.

Spanish often uses the personal a before a specific human direct object, for example:

• Veo a María. = I see María.

But with things, you normally do not use that a:

• Investigó la causa.
• Estudió el problema.

So la causa appears directly after the verb with no preposition.

In standard Peninsular Spanish, a rough guide would be:

la po-li-THÍ-a im-bes-ti-GÓ la KAU-sa

A few useful details:

• c before i in policía is pronounced like th in much of Spain
• investigó is stressed on the last syllable
• causa has the au sound like ow in cow, but shorter and smoother

A more approximate IPA pronunciation is:

[la poliˈθi.a imbestiˈɣo la ˈkau.sa]