PHOENIX (AP) — 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.

An explosive was placed inside the flashlights with a smaller battery and rigged so that turning it on would send an electrical current that triggered the blast, Mangan said. He declined to identify the explosive material.

The first bomb was spotted by a passerby on May 13 in a suburb just west of Phoenix. It was sitting behind a palm tree in a strip mall and blew up when it was clicked on.

The next day, about 10 miles away, a landscaper found a flashlight in an irrigation ditch. It, too, exploded when he flicked the switch, authorities said.

The third bomb exploded on May 24 at a Salvation Army distribution center near downtown Phoenix and about 11 miles from the first one.

An employee detonated the device while sorting through donations, forcing 120 people in the store to evacuate. Jon Bierd, production manager at the facility, said the worker suffered a small abrasion to his forehead.

The Salvation Army stopped accepting donations of flashlights. Since the explosion, employees have not seen any flashlights matching the yellow one seen on the billboards.

"If we have a flashlight that's heavy or is not empty, then I'd call the Phoenix Police Department. No matter where it is, we do not touch it," said Bierd, who is setting aside any flashlight that is donated.

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In addition to the billboards, police are offering a $10,000 reward for tips that lead to an arrest or conviction.

Police have received dozens of calls reporting possible flashlight bombs that either turned out to be false alarms or hoaxes, including one from a Goodwill store.

Meanwhile, the bombings have stopped, though it is unclear whether there are more flashlights out there.

The attention may have scared them off or they may gain confidence and strike again as the investigation stretches on without an arrest, criminal profiler Gregg McCrary said.

Details of the case lead the former FBI agent to think the culprit is either a man or two men, with one of them being a dominant leader and the other a follower.

As for motive, whoever is responsible may be bombing at random for various reasons, said McCrary, who teaches at Marymount University in Virginia.

"Typically these things are about wanting to feel superior and smarter than other people," he said, adding that they also might revel in the news coverage.

"There'll be a vicarious thrill or excitement watching news coverage, and it's kind of like: 'Look what I've done.' It's a sense of empowerment that 'I made all this happen,'" he said.

Mangan said the remnants of the bombs are at a laboratory and being studied for fingerprints and other DNA evidence. The ATF said it will try to trace the materials used in the bombs to see where they were bought.

Mangan said his agency and others are concerned that the bombings will resume, possibly in a different container. They're also worried that the injuries won't be so minor next time.

"Anytime any individual uses a bomb, their purpose is to create fear in the community and also to inflict serious injury or death," he said.

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Associated Press writer Terry Tang contributed to this report.

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