"M.O." is an abbreviation of the Latin phrase modus operandi, which roughly translated means "method of operation". In Criminology, this means the way by which someone has perpetrated/committed a crime.
case
The witnesses and the victims.
The forensic team were at the scene of the crime investigating.
DEBORAH JERMYN has written: 'CRIME WATCHING: INVESTIGATING REAL CRIME TV'
. . . The Forensic Crime Scene Technician or the Investigating Detective.
Chromatography could be used in investigating a crime because it can separate and analyze complex mixtures of substances, such as drugs, poisons, or chemicals found at a crime scene. By identifying and comparing different compounds in samples, chromatography can provide valuable evidence to link suspects, victims, or locations to a crime.
Until the investigating agency has released it and no longer needs it to gather or recover evidence.
"all the people on the train kill him but the ones investigating the crime"
When you investigate crime, it is always possible that the criminal whom you are investigating may violently object.
When the investigating officials have deemed the scene no longer necessary for the collection/preservation of evidence
Act fairly and follow established rules.
what route you took to the scene