Residents in Virginia's Fairfax County should watch for homemade chemical bombs placed in residential mailboxes or on front lawns, authorities say.

Anthony Barrero, battalion chief of the Fairfax County Fire and Rescue Department's Fire and Hazardous Material Investigative Services unit, said 11 such crude devices have been found in Reston, Great Falls and West Springfield since May. No injuries have been reported and the incidents are thought to have been spread by "word of mouth," Barrero said.

"These events tend to occur in clusters," he said. "We suspect they are placed by young adults or juveniles. We don't believe there's any correlation to them other than word of mouth. They're just pranks."

The devices, sometimes housed in small plastic containers like liter soda bottles, typically contain over-the-counter products like drain cleaner or pool chemicals and cause an exothermic reaction, releasing energy in the form of heat. Bits of aluminum foil are also used to provide a corrosive reaction, Barrero said.

"What happens is you take aluminum foil and they cut these [chemicals] up and put it in the bottle and you generate an exothermic effect," he said. "A chemical reaction occurs and the reaction causes the container to swell and blow up."

Barrero said the devices -- commonly known as "works" bombs or "McGyver bombs" -- have been found in mailboxes, on lawns and in wooded areas. Anyone caught in connection to the bombs could face up to five years in prison and a $25,000 fine, he said.

Lee-Alison Sibley, of Great Falls, Va., said she found one such partially-detonated device in her mailbox in June. She suffered a chemical burn to her finger as she reached into the mailbox with a plastic bag around her hand.

"As soon as I moved the bottle, the bottle started to move and sizzle and crackle as there was some kind of acid inside," Sibley told FoxNews.com. "It continued to burn up and smoke -- it was really nasty."

Sibley, who believes she was the victim of a random prank and not a target in the incident, also saw pieces of aluminum foil in the bottle, which she doused immediately and notified authorities.

Sibley's finger swelled to twice its normal size the following day, she said, but the public school teacher hasn't had any lingering health issues since the incident.

"But it's no joke," she said. "It's not funny. What kind of an individual gets pleasure from potentially injuring somebody else?"