Rescue crews didn't have to stumble through every destroyed building in their search for victims after a tornado ravaged a corner of Alabama this week: They used heat-seeking drones to let them know whether there was anyone beneath the ruins.

In so doing, they joined the increasing ranks of public safety agencies across the U.S. and around the world that have employed unmanned thermal-imaging aircraft during critical situations, including manhunts, wildfires and other natural disasters.

In tornado-stricken Alabama, Lee County Sheriff Jay Jones said Tuesday it was not clear if drones spotted an...