Следовательница внимательно посмотрела на улику, но ничего не сказала.

Следовательница is a feminine form meaning female investigator.

It is built from следователь + a feminine suffix. In modern Russian, both can be used for a woman depending on style, context, and speaker preference:

• следователь = the standard job title, often gender-neutral in official usage
• следовательница = explicitly female

So this sentence clearly marks the investigator as a woman.

In the past tense, Russian verbs agree with the subject in gender and number.

Because следовательница is feminine singular, the verbs are also feminine singular:

• посмотрела
• сказала

Compare:

• masculine: посмотрел, сказал
• feminine: посмотрела, сказала
• plural: посмотрели, сказали

So the -ла ending is a very important clue that the subject is feminine.

Russian normally uses смотреть / посмотреть на + accusative for look at something.

So:

• посмотреть на улику = to look at the clue/evidence

You usually cannot just say посмотрела улику with this meaning.

If you want the sense of examine the evidence, Russian would more likely use another verb, such as:

• осмотрела улику
• изучила улику

So на is required here because this is the normal pattern with посмотреть.

Because улика is in the accusative singular.

The dictionary form is:

• улика = nominative singular

But after на with посмотреть, Russian uses the accusative:

• на улику

For feminine nouns ending in -а, the accusative singular usually changes -а to -у:

• улика → улику
• книга → книгу
• девушка → девушку

Внимательно is an adverb meaning carefully, attentively, or closely, depending on context.

Here it describes how she looked at the evidence.

The position is natural and neutral:

• Следовательница внимательно посмотрела на улику

But Russian word order is flexible, so other placements are possible:

• Следовательница посмотрела внимательно на улику
• Внимательно следовательница посмотрела на улику

These are grammatical, but they change the rhythm or emphasis a bit. The original version sounds the most neutral.

Because ничего is the normal form used here in everyday Russian.

A useful practical rule is:

• ничто is mostly used as a subject, often in more formal or literary style
  
  • Ничто не вечно.
• ничего is the common form when nothing is the object of a negated verb
  
  • ничего не сказала
  • ничего не вижу
  • ничего не знаю

So in this sentence, ничего is the normal choice.

Yes, it is a double negative, and in Russian that is standard grammar.

With negative words like:

• никто = nobody
• ничего = nothing
• никогда = never

the verb is also normally negated with не:

• Никто не пришёл.
• Я ничего не понимаю.
• Она никогда не жалуется.

So ничего не сказала is not unusual or emphatic. It is simply the normal Russian way to say it.

They are perfective because the sentence describes completed, one-time actions in a sequence:

• she looked at the evidence
• she said nothing

Посмотрела suggests she took a look / looked and finished.
Не сказала suggests she did not say anything on that occasion.

If you used imperfective forms, the meaning would shift:

• смотрела = was looking / looked for some time
• не говорила = was not speaking / did not speak over a period

So the perfective forms fit a short narrative event very well.

Yes, that is possible.

Both of these are grammatical:

• но ничего не сказала
• но не сказала ничего

The first one, ничего не сказала, is more neutral and more common.

The second one can sound a bit more marked, with slightly more emphasis on nothing as the result.

So for a learner, ничего не сказала is the safest and most natural pattern to remember.