There are a total of four alert levels in the original Destroy All Humans! game:
Alert Level 1: In this alert level, Crypto's presence is revealed to civilians and police officers. Police officers will attack Crypto, as well as farmers.
Alert Level 2: In this alert level, the police will now actively carry out a manhunt for Crypto, and squad cars will patrol the area in search of him. Police officers may now carry shotguns as well.
Alert Level 3: In this alert level, the military is called in. Army trucks and tanks will patrol the town. After high levels of activity and destruction, robots will also be used to attack Crypto.
Alert Level 4: This is the final and thus hardest alert level. Majestic agents will now patrol and investigate the area, and the military will also proceed with their attack. After high levels of activity and destruction, mutants will also be used against Crypto.
The alert levels in later games are similar but slightly different.