Officials also warn that with text messages your approximate location isn't automatically sent to emergency responders, like it is with voice calls. Instead, they encourage people to give 911 call takers an accurate address or location as quickly as possible.
Supporters of such systems say their use would go beyond active-shooter and hostage situations to scenarios in which a battered spouse, for example, could surreptitiously message police without alerting the attacker.
"If someone could snap a photo or a quick video showing the perpetrator that'd be enormously helpful to law enforcement," said Joseph Giacalone, a criminal justice professor at John Jay College of Criminal Justice and a retired police detective.
San Bernardino, California, rolled out its text-to-911 service in December about two weeks after an attack at a social services center where a man and his wife killed 14 people at a holiday gathering. In New Hampshire, where text-to-911 service is available statewide, Democratic Gov. Maggie Hassan said it was a "common-sense initiative that will help save lives."
Text-to-911 service also has been used by deaf and hard-of-hearing people to get in touch with police.
A deaf woman in Alpharetta, Georgia, texted police to report there were two children locked in a car in a shopping mall parking lot, and police rescued them.
Authorities say 911 texting, like its phone counterpart, has also been abused.
Last year, a teenage girl texted 911 to falsely report there was an active shooter at a high school in Marietta, Georgia, said police, who arrested her at her home an hour later.