Do not attempt to move any human remains you find, if you encounter the actual body of a deceased human being in public or anywhere else. Instead, note the precise location of the remains, get to a phone as soon as possible and notify your law enforcement officials.
List the reasons you've come to suspect a person you're acquainted with of murder, if you're in a situation where you think a violent crime may have been committed and covered up. The most likely explanation is the probable one, especially if the suspected victim is known to be missing.
Contact your local police department by telephone. Tell the officer you speak to about every detail of the suspected crime that you can bring to mind. Trust the officer's judgment as to whether or not it's likely that a murder has actually taken place, and act according to the instructions the police officer gives you.
Notify the Federal Bureau of Investigation as an alternative to the local police. Keep in mind, though, that the different law enforcement jurisdictions (municipal/state and federal) may wind up complicating the investigation of the suspected case.
Report the suspected murder anonymously through a crime tip hotline in your local area. Many people prefer to remain anonymous when reporting suspected crimes to ease any fears they may have about the suspected perpetrators finding out who called the police.
Consider coming to the aid of the prosecution if the police have laid charges and a criminal trial is pending, should you have evidence that might help a jury convict the suspect.