What's a Blue Alert and why did my phone just go off?

You've heard of Silver and Amber Alerts, but what is a Blue Alert?

TEXAS, USA — As law enforcement searches for a suspect accused of killing a Nassau Bay police sergeant, millions of Texans' phones notified them to be on the lookout with a Blue Alert.

The Blue Alert was launched in Texas by former governor Rick Perry in 2008.

It was created to let residents across the state know when an offender who killed or serious injured a federal, state or local law enforcement officer in the line of duty is on the run.

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

Wednesday's alert was an effort to locate 21-year-old Tavores D. Henderson, who is accused of running over Nassau Bay Police Sgt. Kaila Sullivan, 43, during a traffic stop Tuesday night.

Police said she was assisting another officer with an arrest when she was struck and killed

There is a reward of up to $20,000 for information leading to Henderson's arrest.

If you have any information regarding Henderson's whereabouts, call (713) 222-TIPS.

What is the meaning of blue alert?