What is the ethical obligation of a private investigator if they observe a serious crime being committed in front of them while on assignment?

1. To take personal action to apprehend the perpetrators themselves.
2. To ignore the crime, as their primary duty is to their client and the investigation.
3. To intervene if safe and possible, and immediately report the crime to the appropriate law enforcement authorities.
4. To continue their surveillance, document the crime, and only report it to their client.