Authorities are issuing a public warning after they say three homemade explosive devices were recently found in Worcester, Massachusetts.

The Worcester District Attorney's Office issued a warning Tuesday morning alerting residents to be on the lookout and inform authorities if they see unattended coolers or other suspicious items.

The first device was found on June 19, when a small explosion occurred on Madison Street in Worcester. Investigators said they found fireworks, a butane gas can and projectile materials at the scene. Two other devices that were not detonated but were built with similar materials and placed inside coolers, were also found in Worcester last month.

"As authorities continue to investigate, they encourage the public to be mindful of unattended items, particularly coolers," the district attorney's office said in a statement. "Do not attempt to tamper with these items. Instead, call the authorities immediately."

"Massachusetts residents have caused themselves grievous injuries while attempting to create homemade explosive devices," State Fire Marshal Jon M. Davine said. "These devices are illegal and potentially fatal."

Anyone who sees any suspicious items are asked to call 911.