TEXAS (KXAN) — You hear the alert comes across your phone with that emergency tone or constant buzz, the big yellow triangle pops up and it says "Blue Alert." What does that mean? Blue Alert issued after Harris County deputy shooting The Blue Alert program is used to alert the public about people who are accused of killing or seriously injuring police officers or other members of law enforcement, in an effort to apprehend the suspect as soon as possible, according to Texas DPS. Former Texas Gov. Rick Perry signed an executive order in 2009 to start the program, which includes emergency messages sent to cell phones and other devices with relevant information about the suspect designed to facilitate tips and leads. The criteria DPS uses to issue the alerts are: 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 DPS A sign of the times: Balloon Fiesta mixes tradition with high-tech drones Texas DPS originally sent the alert to cell phones, the @TX_Alerts Twitter account and email. To subscribe to alert emails, go to the DPS' site and sign up. Emergency alerts can be toggled on and off in your cell phone settings.