(CNN) -- A pipe bomb hidden inside a newspaper exploded when an elderly Vacaville, California, man picked up the paper near his doorstep Sunday morning, ATF spokesman Marc Willis said.

The man, who suffered "certainly serious, but not life-threatening injuries," was airlifted to a hospital, Vacaville city spokesman Mark Mazzaferro said.

Residents of a dozen neighboring homes were evacuated for several hours, but they were allowed to return home later Sunday afternoon, Mazzaferro said.

Investigators were questioning neighbors and relatives to determine why the man may have been targeted by a bomb, he said. It was not immediately known if the victim was targeted.

"Neighbors are telling authorities that an unidentified man in camouflage was seen in the neighborhood last night," Willis said.

A bomb squad from Travis Air Force Base and federal investigators combed the neighborhood with explosives-sniffing dogs in the Brown's Valley section of the town, but they found no other explosive devices, Mazzaferro said