The manhunt for Bradley William Stone, 35, focused in and around Pennsburg, Pennsylvania, where residents were advised to remain indoors with their doors locked, the Montgomery County district attorney said.
Stone, who lives in Pennsburg, was a suspect in the shooting deaths on Monday (local time) of his ex-wife, his former mother-in-law, former grandmother-in-law, former sister-in-law and two other family members, District Attorney Risa Vetri Ferman told a news conference.
"As we stand here right now, we don't know where he is," Ferman said, adding that Stone took his two young daughters from the home of his ex-wife but left them with a neighbor and they were safe. Pennsburg is about 80km northwest of Philadelphia.
Ferman said Stone sometimes uses a cane or walker, but it was not clear if he actually needed such a device to get around.
As night fell on Monday, the search for Stone turned to neighboring Bucks County, where a man walking his dog was confronted by a suspect carrying a knife who tried to take his car keys.
The dog walker was armed, and fired several shots at the suspect. Police in Doylestown Township, about 24 miles from Pennsburg, say the knife-wielding man fit Stone's description.
Media reported that Stone suffers from post-traumatic stress disorder. NBC said he had enlisted in the US Marine Corps Reserves in 2002 and was honorably discharged in 2011.
Stone was deployed at least once to Iraq, NBC said, adding that his military records did not show that he had been wounded in combat.
Stone was believed to have gone on a shooting spree in three towns, first killing his ex-wife's sister, husband and 14-year-old daughter in Souderton at about 3:30 a.m. ET, the district attorney said. A 17-year-old son survived but was wounded and hospitalized, she said.
About an hour later, police received a hang-up call to 911 pointing them to a home in Lansdale, Ferman said. There they found two dead women - Stone's former mother-in-law and former grandmother-in-law, she said.
About a half hour later, police received a 911 call from a neighbor of Stone's ex-wife, Nicole, in Lower Salford, Ferman said. Police found the 33-year-old ex-wife shot to death, she said.
