What is 'Blue Alert' and why are people receiving it on their phones?

Millions of smartphone users learned through the Blue Alert that a suspect accused of killing a police office is on the loose.

According to police, 21-year-old Tavores D. Henderson is accused of killing Nassau Bay Police Sergeant Kaila Sullivan, 43, Tuesday night.

The suspect managed to escape as he was being handcuffed. The deceased police officer was hit by the suspect’s vehicle when he fled the scene.

Authorities have announced a reward of up to $20,000 for information leading to Henderson's arrest.

Launched in Texas in 2008, chief purpose of the Blue Alert was to let the people across the state know when an offender who killed or seriously injured a federal, state or local law enforcement officer in the line of duty is on the run.

There are four criteria that must be met to issue the alert:

A law enforcement officer must have been killed or seriously injured by an offender.

The investigating law enforcement agency must determine that the offender poses a serious risk or threat to the public and other law enforcement personnel.

A detailed description of the offender's vehicle, vehicle tag, or partial tag must be available for broadcast to the public.

The investigating law enforcement agency of jurisdiction must recommend activation of the Blue Alert to the Texas Department of Public Safety (DPS).

Texas is one of the 34 states that currently has the system in place.