Days after charging a second suspect in a series of mysterious explosions that rocked upper Bucks County this spring, law enforcement officials are warning residents to be wary of additional improvised explosive devices they may have left behind.

"As always, our number one priority is to make sure everyone in Bucks County is safe, and we are sharing this information with that priority in mind. This is more an FYI than an SOS," District Attorney Matt Weintraub said in a statement Tuesday. "Nonetheless, we will continue to do our best to protect you by working hard to find any unexploded devices that may remain. If you find something suspicious, please do not investigate it on your own. Call 911."

David Surman Jr., 30, has been charged with building and deploying these homemade explosives throughout the northern, rural portion of the county, an hour north of Philadelphia. His girlfriend, Tina Smith, 31, was arrested Friday and charged with helping him distribute the explosives.

At Smith's arraignment, prosecutors said evidence linked Surman and Smith to seven blast sites around the county. An additional five explosives, including one that was 18 inches long, were recovered in June from the home Surman and Smith shared in Quakertown.

A task force of local, state, and federal authorities are continuing their probe into the explosions, which were reported between April and June, usually at night in remote, sparsely populated areas. Investigators are concentrating current efforts on finding leftover explosives in "the greater Quakertown area, especially Milford Township, as well as locations near Ottsville and Upper Black Eddy," according to the district attorney's office.

Police were led to Surman, owner of Consolidated Chemicals & Solvents, after a state trooper spotted him and Smith fleeing the scene of one explosion site. Text messages between the two, revealed in court documents, showed they laughed about an explosive device that detonated underneath a public works employee's lawn mower in Milford Township.

Surman faces trial in the case in February. Also, he has a hearing next month on charges in connection with child pornography that investigators allegedly discovered while seeking evidence related to the explosions. Smith faces a preliminary hearing Nov. 30.

Both were freed after posting bail, court records show.