Final answer:
The enormous amount of violence connected to the St. Valentine's Day Massacre most likely spurred authorities into action to shut down the mob bosses in Chicago.
Step-by-step explanation:
The event that most likely spurred authorities into action after the St. Valentine's Day Massacre was the enormous amount of violence connected to this event. The St. Valentine's Day Massacre was a brutal and highly publicized gangland murder that resulted in the death of seven members of a rival gang. The level of violence and the audacity of the crime shocked the public and law enforcement, prompting the FBI to increase resources to bring down the mob bosses in Chicago.