Late Monday, investigators pursued a complaint that Stone might have been spotted in Doylestown, according to a law enforcement source. Stone was "on the run," the source said.
At the same time, Pennsburg activated a "shelter in place" recommendation. "We ask that all residents please remain in their homes with doors locked and away from any windows," the alert said. "Please do not answer your door for anyone other than law enforcement officers."
The Upper Perkiomen School District canceled Tuesday classes, citing the search for Stone.
Prosecutors did not identify Stone as the suspect until midafternoon and then released only sparse details, saying he was believed to be armed, had a red or auburn beard, and could be wearing fatigues and using a cane or a walker. Hours later, District Attorney Risa Vetri Ferman's office said Stone might have shaved off his beard and might not use a walker.
At a brief news conference outside the Lower Salford Township police building, Ferman did not discuss a motive and declined to take questions. "We do not know where he is," she said.
County court records show that Stone and his wife, Nicole A. Stone, 33, began divorce proceedings in early 2009. The divorce was finalized, but battles over custody of their two daughters, believed to be teens, were ongoing. Early this month, he filed an emergency motion seeking custody.
Ferman said that after the killings, Bradley Stone dropped the couple's two daughters off with a neighbor in Pennsburg.
Lisa Andrey, a longtime neighbor to Nicole Stone's mother, said the couple had "a very volatile relationship" that deteriorated after the Marine returned from serving in Iraq.
When the pair first got together "he was a great guy and an excellent father," Andrey said. "And then he went away to Iraq and came back and was a completely different man."
In a statement, the Marine Corps confirmed that Stone served between 2002 and 2008. His four service medals included the Global War on Terrorism Service Medal and the Iraq Campaign Medal.
Among the Facebook groups that he appeared to list among his favorites were pages for local and national veterans groups and the National Center for Post Traumatic Stress Disorder. Court records also show Stone had three DUI arrests, and was on probation for one conviction.
Ferman said the first shootings occurred about 3:30 a.m. in Souderton, where Patricia Flick, the suspect's former sister-in-law, her husband, and her 14-year-old daughter were killed. Flick's 17-year-old son was shot and was being treated at a Philadelphia hospital, the prosecutor said.
About an hour later, Mary and Henry Virgilio said, they were awakened by "police lights shining in the bedroom" of their home on the 100 block of West Fifth Street in Lansdale. Across the street, they saw officers in the house where Nicole Stone's mother and grandmother lived and were killed.
Then police were summoned to the 100 block of Main Street in Harleysville, where Nicole Stone was found dead.
At each location, police scrambled to secure the scene.
Judy Alderfer, a restaurant manager who lives in Souderton, said that police there had ordered residents to leave their houses, and that they took shelter in a nearby McDonald's.
Audrey Gallina said that when she came out of her house, officers warned her: "There's a man with a gun, he's very dangerous, go inside."
The incidents also touched off a "shelter in place" order at all schools in the Souderton Area School District. Students in the neighboring Upper Perkiomen School District were forced to remain in classes after the usual dismissal time.
By early evening Monday, police tactical teams had surrounded a twin home in Pennsburg, where the front door evidently had been smashed.
But a raid there yielded nothing.
"Nothing like this ever happens here," said resident Luke Shimpeno, 25. He described his street as a "nice neighborhood, where kids are safe and can ride their bikes around."
In Lansdale, Andrey said he was heartbroken for the couple's daughters.
"Nicole gave her entire life to her baby girls," she said, "and he took everything away from those babies - everyone that they knew, and now they have no one."
Visit Philly.com for the latest developments in the investigation and manhunt.
Contributing to this article were Inquirer staff writers Anthony R. Wood, Craig R. McCoy, Kathy Boccella, Chris Palmer, and Jeremy Roebuck.