1. Criminal charges are filed
A criminal case typically starts when the prosecution (usually a district attorney) files charges in court. This is called a Complaint. A Complaint says what crimes someone is charged with. The prosecutor files a Complaint after reviewing a police report.
This describes the typical way charges are filed. It's possible for charges to be filed another way.
How a case starts
Prosecutors get a report from law enforcement
Law enforcement writes a report that describes what information they have about whether someone committed a crime. It will include what they saw or heard, what evidence they've collected, and what any witnesses said. If they have it, it will include pictures, videos, transcripts of recorded statements, and lab reports. It usually says what charges, for what crimes, they recommend the district attorney should file.
The prosecutor decides whether to file charges and, if so, what charges to file
The prosecutor reviews the police report. Then, they decide whether to file charges. They can also ask the police to investigate more. If they file charges, they decide whether to file misdemeanor or felony charges.
In general, someone can only be kept in jail for 48 hours, not including days the court is closed, without being charged with a crime. If they aren't charged by the deadline, they must be released. The prosecutor might file charges later. But, the person cannot be kept in jail if they are not charged with a crime.
The charges filed are called a Complaint. A Complaint lists the crimes the defendant is accused of committing and when. A defendant has a right to a copy of the Complaint. It may be mailed or handed to them. They will get a copy on their first court date.
What's next?
If the prosecutor decides to file charges, then the defendant will have an arraignment.