There were a total of 4 Alert Levels in Destroy All Humans!:
Alert Level 2: In that alert level, the Police actively carried out a manhunt for Crypto, and squad cars patrolled the area in search of him. Police officers carried Shotguns as well.
Alert Level 4: The final and thus hardest alert level. Majestic Agents patrolled and investigated the area, and the Army also continued with their attack. After high levels of activity and destruction, PSI Mutants were also used against Crypto.
The alert levels in later games were similar but slightly different.