ABC News' Pierre Thomas and Toni Wilson report:

Three discarded flashlights-turned-bombs have exploded in the past month in the Phoenix area, injuring five people and prompting police to warn the public about the potential danger of discarded flashlights.

Sources tell ABC News they fear that whoever is making the devices will eventually increase the power of the improvised bombs, and someone might die.

Janelle McKee was at a Glendale, Ariz., strip mall May 13 when she noticed a yellow flashlight sitting by a palm tree. She picked it up, and when she turned it on, it exploded.

"It sounded like a shotgun, big loud boom," McKee said, adding, "I definitely won't be picking anything up off the ground anytime soon."

A day later, a landscaper found a flashlight in a ditch and he, too, turned it on. The device injured two people when it exploded.

There was another explosion May 24 at a Salvation Army distribution center near downtown Phoenix. Two people were injured.

"We often get very strange things that are donated, but we never get things that are donated with the purpose to do harm," the Salvation Army's Capt. John Desplancke said.

Authorities believe the alleged bomber has spent time thinking about the devices because the bombs have been placed in an object that people would instinctively turn on.

Authorities say the devices have fairly sophisticated circuitry. The device is triggered when the flashlight is turned on, and the battery emits an electrical charge that ignites an explosive.

"Are we concerned that since there has been more than one? Absolutely, we're concerned," Tom Atteberry, special agent in charge at the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives in Phoenix, said.

"We do not want an innocent child or victim to pick one of these flashlights up and get injured or killed, so we take this very seriously," Atteberry added.

Authorities have put up 22 billboards in the Phoenix area to warn residents that yellow flashlights could be dangerous and should not be touched.

Police believe the same person is responsible for all three attacks because the three flashlights that exploded had the same design and the chemical explosives.

The recovered flashlight bombs have been sent to a lab for testing.

"Our focus is not only to apprehend the person or persons responsible for these three incidents but to heighten the awareness of the public," Thomas Mangan, special agent for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives told ABC News. "I am certain that someone knows who is responsible for these acts of violence and we encourage them to come forward and contact us before someone is seriously hurt."

The ATF is offering a $10,000 reward for information leading to an arrest and conviction of the person responsible for the flashlight bombs. Anyone with information is asked to call 1-888-ATF-BOMB (1-888-283-2662).