The first thing that should be done when an intrusion into an organization network is detected is to identify nodes that have been compromised. Identifying nodes that have been compromised is a critical step in responding to an intrusion, as it helps determine the scope, impact, and source of the attack, and enables the implementation of appropriate containment and recovery measures. The other options are not the first things that should be done when an intrusion into an organization network is detected, as they may be premature or ineffective without identifying nodes that have been compromised. Blocking all compromised network nodes is a containment measure that can help isolate and prevent the spread of the attack, but it may not be possible or feasible without identifying nodes that have been compromised. Contacting law enforcement is a reporting measure that can help seek external assistance and comply with legal obligations, but it may not be necessary or appropriate without identifying nodes that have been compromised. Notifying senior management is a communication measure that can help inform and escalate the incident, but it may not be urgent or accurate without identifying nodes that have been compromised. References: CISA Review Manual (Digital Version), Chapter 4, Section 4.2.2