Final answer:
The term describing the point in an investigation when a suspect can't deny involvement is termed as 'inculpatory evidence' or 'turning point'. This implies such strong evidence against the suspect that a denial is implausible.
Step-by-step explanation:
The term you are looking for that describes a point in an investigation during which the suspect cannot deny his involvement is known as inculpatory evidence or a turning point. This is a stage where the evidence gathered and presented by law enforcement is so compelling that it directly ties the suspect to the crime, effectively reducing or eliminating the possibility of a denial. In the scenario provided, of the two individuals apprehended, the moment one defects and provides information that indicates the other's guilt in the armed robbery, it could represent such inculpatory evidence.
However, it's important to bear in mind that in any criminal investigation, the presumption of innocence is central, and the accused is considered innocent until proven guilty in a court of law. Police techniques, such as separating suspects to prevent collusion and suggestive questioning, can sometimes lead to false admissions or confessions. Moreover, the right against self-incrimination is a fundamental legal safeguard designed to protect suspects from being compelled to testify against themselves.