Learn Swedish: Gravarna blev faktiskt noga undersökta. - The graves were actually carefully examined.

Because it is a past participle used in a bli passive sentence. In this type of passive, the participle acts like an adjective and has to agree with the subject. Since Gravarna (the graves) is plural, the participle takes the plural ending -a instead of the standard singular form undersökt.

Could I write Gravarna undersöktes instead of Gravarna blev undersökta?

Yes, both translate to 'The graves were examined.' The s-passive (undersöktes) focuses purely on the action taking place. The bli passive (blev undersökta) focuses slightly more on the transition or the result of the action. The biggest grammatical difference for you to remember is that the bli passive requires the participle to change its ending to match the plural subject.

Why are the words faktiskt noga placed right in the middle of blev and undersökta?

In a Swedish main clause, the first verb is always in the second position (the V2 rule). Here, blev is that first verb, so a sentence adverb like faktiskt (actually) will usually come right after it. The descriptive adverb noga (carefully) is placed immediately before undersökta because it directly describes how the examining was done.

I've seen noggrant used for 'carefully'. Why does this sentence use noga?