"It was premeditated, it was calculated, it was planned," Allegheny County District Attorney Stephen Zappala told reporters Thursday in the borough just east of Pittsburgh. "It's one of the most brutal I've seen. I've been the DA for 18 years; I haven't seen something like this."
The suspects remain at large, and authorities haven't identified them. Zappala said police talked to one victim at the hospital, but "we haven't developed that enough to give some definite detail to (describe a gunman's) face."
The first shots rang out from an alley behind a house in the Pittsburgh suburb shortly before 11 p.m. Wednesday. Partygoers tried to escape through the back of the house -- only to have someone open fire from a yard adjacent to the residence, police said.
According to Zappala, a man armed with a handgun first approached those at the barbecue and led them "a relatively short distance along (a) fence" until they were in close range of a second suspect "waiting with an AK-47-type weapon."
"It wasn't like he was just squeezing shots off randomly," the prosecutor said. "They were all head shots."
Neighbor Jackie Johnson recalled hearing a constant rat-a-tat of about 30 bullets.
"When I came out, people were screaming and running," Johnson told CNN affiliate KDKA-TV
. "And bodies were laying on the ... porch."
2 remain in critical condition
Allegheny County authorities said three of the six slain were siblings -- Jerry Shelton, Chanetta Powell and Brittany Powell. Chanetta Powell was eight months pregnant, Allegheny County police Superintendent Charles Moffatt told reporters. Counted among the six slain was her 8-month-old fetus. The other two killed were Tina Shelton and Shada Mahone.
Three people were wounded. Two men were in critical condition, while one woman had already been treated and released.
Most of the victims suffered multiple wounds, Moffatt said. Authorities recovered 48 shell casings.
"All of them, they would light up a room. They loved to laugh and joke and have fun," Jessica Shelton, who lost three children and two nieces in the violence, told reporters through tears.
"And for these shooters out here ... you just didn't take innocent lives, you made children lose their parents. You got to stop it now," she said.
The district attorney indicated at least one child was in the residence at the time. Several people managed to escape unharmed.
Ballistic tests indicate the gunmen escaped on foot, with no details offered on where they went, how they knew the victims (if at all) or whether they eventually used a getaway car.
Zappala said retaliation may be a motive or there may be some sort of drug connection.
"There's no indication that this was either a stash house or they were running an operation out of here," he added. "But drugs are definitely in play."
The Bureau of Alcohol, Tobacco, Firearms and Explosives is offering a reward of up to $20,000 for information leading to the arrest and conviction of those responsible for the shooting.
In video from CNN affiliate WPXI-TV of the shooting aftermath, a woman approached the house and cried out, "I've lost my baby."
Another woman grappled to make sense not only of the carnage but also that it happened in her neighborhood.
"This street is always quiet," she told the TV station. "There's nothing but kids on this street."
Governor: 'Must find better ways to stop gun violence'
Zappala said the area of Wednesday night's shooting hadn't seen a lot of crime but other parts of the borough of about 15,000 people had.
"I think we need to bring more assets to bear" in Wilkinsburg, he said. "I'd like to see something more proactive."
The mass shooting spurred some Pennsylvania politicians, including Sen. Bob Casey and Gov. Tom Wolf, both Democrats, to urge more to be done to stop gun violence.
"Communities like Wilkinsburg -- and other towns and cities across Pennsylvania like Pittsburgh, York, where I live, and Philadelphia -- know all too well the horrific nature of gun violence in their neighborhoods," Wolf said.
"We must find better ways to stop gun violence and to keep guns out of the hands of criminals."
Johnson similarly had trouble digesting what happened next door to her in Wilkinsburg.
"People's lives taken like that, just on the drop of a hat," she told KDKA. "It's just insane.
"And it needs to stop. It has to stop."