Learn Hardcore Japanese: ziko no ato keisatu ga kimasita.

Because あと here is a relational noun meaning “after.” To link two nouns (事故 + あと), Japanese uses の, so 事故のあと literally means “the after of the accident,” i.e. “after the accident.”

Why isn't there a particle like に after あと?

With temporal nouns such as あと, 前, 中, etc., the particle に is optional. Both of these are correct and mean the same thing:

事故のあと警察が来ました。
事故のあとに警察が来ました。

Can あと be written in kanji?

Yes. You can write 事故の後 instead of 事故のあと. Writing in kana is often preferred for readability, but using the kanji 後 is perfectly acceptable.

Why is 警察 marked with が instead of は?

が marks the subject or introduces new information—in this case “the police” as the ones who came. Using は would make it a topic and often implies contrast or emphasis (“as for the police…”), which isn’t necessary here.

Could we use は instead of が?

Yes, but the nuance shifts.

事故のあと、警察が来ました。 (Neutral statement: “The police came after the accident.”)
事故のあと、警察は来ました。 (Implying contrast: e.g. “The police did come after the accident [but someone else didn’t].”)

Why is the time phrase 事故のあと placed at the beginning of the sentence?

In Japanese, time and place expressions typically appear before the subject and verb to set the scene. So starting with 事故のあと (“after the accident”) is natural word order.

Why is the verb 来ました in the polite past form?

来ました is the polite past tense of 来る (“to come”). In casual speech you could use the plain past 来た.

Polite: 事故のあと警察が来ました。
Plain: 事故のあと警察が来た。

Can I drop 警察 and just say 事故のあと来ました?

Generally you need a subject or topic for 来ました to be clear. Japanese does allow dropping subjects when context is obvious, but 事故のあと来ました sounds ungrammatical because who “came” isn’t specified. Keeping 警察が来ました avoids confusion.

Is there any difference between writing 事故のあと and 事故後?

Yes.
• 事故のあと uses の + kana あと, sounding more conversational.
• 事故後 (with kanji and no の) is more formal or technical (e.g. in reports or written notices). Their meanings are the same, but the style differs.