By AMANDA LEE MYERS

Ross D. Franklin  /  AP

This is one of 22 such billboards across the Phoenix metro area warning citizens about picking up discarded flashlights.

PHOENIX — Flick the switch on these flashlights and they don't light up. They blow up.

Three of these bombs have exploded within the last month in the Phoenix area, causing minor injuries to five people and raising fears of more serious ones.

Police still have no idea who is behind them and have taken the unusual step of putting up 22 billboards across the sprawling metro area to warn residents about discarded flashlights.

"The nature of the bombings are so random," said Tom Mangan, a special agent at the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives in Phoenix.

Mangan said the agency has ruled out any connection to terrorism because the targets have been random and there have been no messages or demands.

The ATF said the bombs appear to have been made by the same person or people because their design was identical.