Armed with a .45-caliber handgun, the man ran from the house carrying a black leather bag under his left arm "like a football," homicide Detective Philip Nordo said.
Police described the man as of medium build, about 5-foot-7, with dark, spiky hair and wearing an untucked royal blue, long-sleeve dress shirt, black dress pants, and black dress shoes.
The shooter ran down Westbury toward Malvern Avenue, and then toward Haverford Avenue, where police believe he had a getaway car, Nordo said.
Police have looked at surveillance-camera footage from the area and are interviewing residents who saw the fleeing man.
There is a $40,000 reward in the killings. Tipsters are asked to should call homicide detectives at 215-686-3263.
The shootings shocked many in the mostly working-class neighborhood of red-brick rowhouses and tidy lawns bordered by the well-heeled townships of Haverford and Lower Merion.
Police arrested the boys' parents, Rohan and Cynthia Bennett, after discovering a large stash of prescription pills, handguns, prescription pads, and $112,000 cash in their bedroom.
Police called the parents higher-end drug dealers trying to conceal their criminal activity behind the veil of a quiet neighborhood.
Neighbors did not report seeing any suspicious activity at the house.
Malcolm, the older of the brothers, had three previous drug arrests. He was shot once in the chest.
Rohan Bennett, a straight-A student who had just finished middle school, was an innocent bystander, Nordo said.
He was shot four or five times.
"He's definitely not a target," Nordo said. "He was just caught up in this barrage of bullets being fired in the middle of his living room."
An 18-year-old friend of the boys' was also in the house but managed to run for cover.
The friend was also arrested on drug charges but has been released on bail, and police say they cannot find him. Investigators said they did not consider him to have been involved in the shooting.
"These were his two best friends killed directly in front of him," Nordo said. "We are trying to open up a line of communication with him. We would expect him to cooperate."
Bennett was upstairs when his sons were shot. Cynthia Bennett was running an errand with her 12-year-old son at the time of the shooting.
Police believed that someone let the killer inside and that an argument ensued. Then came the gunfire.
Contact Mike Newall at 215-854-2759 or firstname.lastname@example.org.