Authorities say they have someone they would like to speak with in connection to the shooting

'The murders were planned. They were calculated, brutal,' District Attorney Stephen Zappala said

The reason for the shooting is not known - but police say they haven't ruled out drugs as a motivating factor

Two men remain in critical condition and a 41-year-old was treated and

Chanetta Powell was pregnant and due to give birth in May; the death of her child has been ruled a homicide

Siblings Jerry Shelton, 35; Brittany Powell, 27; and Chanetta Powell, 25; were shot dead at a cookout at Brittany's home

Police are searching for at least two gunmen who opened fire at a Pittsburgh, Pennsylvania cookout late Wednesday night

Authorities say the death of an 8-month-old male fetus in a shooting at a backyard cookout near Pittsburgh has been ruled a homicide.

That brings the number of homicides in the late Wednesday night ambush to six.

The dead included three siblings, Brittany Powell, 27, who lived at the home; Jerry Shelton, 35; and Chanetta Powell, 25. The other two were Shada Mahone, 26, and Tina Shelton, 37.

Chanetta Powell was pregnant and due to give birth in May.

The gunmen working as a team fatally shot five people and critically wounded two others at the backyard cookout, with one attacker using a high-velocity rifle to shoot all the victims in the head as they were driven in his direction, a prosecutor said Thursday.

'The murders were planned. They were calculated, brutal,' District Attorney Stephen Zappala said.

The gunmen appeared to have targeted one or two of the victims, said Zappala, who added that they hadn't ruled out drugs as a motive.

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Chanetta Powell (left) and Brittany Powell (right) are two of the five people who were killed Wednesday in a shooting at a Pittsburgh cookout. The two women were sisters

Victims Jerry Shelton (left), Chanetta Powell and Brittany Powell were siblings. Jerry Shelton and Tina Shelton (right) were cousins

Above, the fifth victim Shana Mahone. Four of the victims were killed on the back porch and another died at the hospital

The victims killed as they rushed toward the back porch and an alleyway to seek cover as a gunman fired a .40-caliber pistol at as many as 15 adults who were playing cards and having a late-night cookout in the yard.

That steered the victims toward the rear porch and door of the house, where an accomplice armed with a 7.62 mm rifle similar to an AK-47 shot them from behind a chain-link fence less than 10 feet from the porch, Zappala said.

Neither weapon has been found

The man with the rifle aimed high throughout the barrage of bullets. Four of the dead were found on the tiny back porch.

'They were all head shots,' Zappala said.

The mother of Brittany and Chanetta Powell and Jerry Shelton says her family was massacred.

A tearful Jessica Shelton said Thursday that it makes no sense for anyone 'to take people's lives like that.'

Mrs Shelton added that there were three men at the party that she wasn't close with, and that she's suspicious about the fact they escaped unharmed.

She says the other two shot dead were her nieces. Another of her sons' was critically wounded.

'It looks like right now they were all fleeing toward the backdoor of the residence when the second gunman fired from the side of the yard,' Lt. Andrew Schurman of the Allegheny County homicide unit said. 'They all seemed to get caught on the back porch.'

Jessica Shelton's daughter Chanetta and Brittany and son Jerry were killed in the shooting. She is pictured above at a Thursday press conference

The shooting took place in the backyard of Brittany's home, and friends and family were gathered for a cookout. Above, the home on Thursday

Three of the victims were siblings and two of the victims were cousins. Authorities say one of the women was pregnant

Allegheny County detectives look over the scene of a shooting on Thursday in Wilkinsburg, Pennsylvania

Five people were killed and three people injured when two gunmen attacked a backyard barbecue in suburban Pittsburgh, Pennsylvania late Wednesday

Police are now searching for the two gunmen behind the 'ambush-style' attack

Four people died at the scene and another died at the hospital. Two men are in critical condition and a 41-year-old woman was treated and released for her injuries

A woman reacts at the scene of the deadly shooting in Wilkinsburg early Thursday morning

Police have not released a motive for the crime yet. Above, members of the community react at the scene

Members of the community react at the scene of a deadly shooting in Wilkinsburg, Pennsylvania on Thursday

Schurman says the gunmen barged into the party and opened fire in an ambush-style attack.

He said gunshots came from two distinct locations but police don't believe anyone at the party fired back.

Tina Shelton's father spoke out to WTAE saying she was a 'good woman'.

'Everybody that knew her loved her. She was an innocent bystander,' Vernes Pugh said, adding that his daughter leaves behind three children.

Resident Kayla Alexander tells WPXI-TV that she heard a barrage of gunshots - more than 20 - in the neighborhood which usually is quiet.

Groups of residents gathered on the street, some of them sobbing and saying they lost family members.

'My baby! My baby!' one woman at the scene could be heard screaming.

While another woman sat crying on the ground added: 'They just wanted to kill everybody.'

Authorities say the people at the party fled to the backdoor of the house when the second gunman started firing from the side of the yard

One gunman was shooting from a yard while another came up an alley, witnesses told police

Resident Kayla Alexander told WPXI-TV that she heard a barrage of gunshots - more than 20 - in the neighborhood which usually is quiet

Wilkinsburg is a poorer, largely blighted suburb just east of Pittsburgh that is known for drug trafficking and gun violence

Carl Morris and his son, Robert, were getting ready to leave their house across the street when they heard a volley of three shots, a pause, then gunfire lasting more than a minute.

Robert Morris said he saw children run onto the small back porch and scream, 'Mommy, Mommy'.

'It was terrible,' the younger Morris said.

The Morrises said a woman in her 20s or 30s lived in the home with at least one daughter. They said the house was considered a 'safe haven' in the neighborhood.

The backyard is about 30 feet by 50 feet. Police said they found one pile of shell casings just outside the yard in an alley. They found more shells along a fence that separates the house from a neighbor's yard, which means a second gunman was less than 10 feet away from the porch where victims ran for cover.

Bullet holes were visible Thursday around the porch addition. Tables and chairs, some tipped over, remained in the backyard, signs of a party quickly abandoned.

Police respond to the scene of the deadly shooting in Wilkinsburg, Pennsylvania on Thursday

A police officer responds to the scene of a deadly shooting in Wilkinsburg, Pennsylvania on Thursday

Wilkinsburg is a poorer, largely blighted suburb just east of Pittsburgh that is known for drug trafficking and gun violence. But, neighbors described the street on which the shooting occurred as generally quiet and expressed shock.

James Boyd, a neighbor who lived three houses down from the shooting said he was alerted by the sound of gunshots.

He told the Pittsburgh Post Gazette: 'We've had trouble in this neighborhood before but never this close to home. I've never seen anything like this before.'

While another woman who lives on the street added: 'This street is always quiet, there's nothing but kids on this street. To see that, it's new, and it's probably shocking to everyone else up here. I'm shaking.'

Vehicles from the medical examiner's office arrived in the neighborhood early on Thursday to remove the victims' bodies. Above, ambulances at the scene

Vehicles from the medical examiner's office arrived in the neighborhood early on Thursday to remove the victims' bodies.

Preliminary reports say that the incident may have started at a party in a backyard in Wilkinsburg on the 1300 block of Franklin Avenue at around 11:00pm, according to WTAE.

Several police departments were on scene, including Allegheny County homicide detectives.

In the wake of hte shooting, County Executive Rich Fitzgerald released a statement expressing his sympathies to the victims and their families.

'We know that these families' lives have been changed forever. Over the next few days, I'm certain we will learn more about each of the victims who died and their futures, now cut short. Wilkinsburg is a community filled with grief, shock and anger this morning. We share their grief and offer them our support in the days and weeks to come,' he said.

Anyone with information is being asked to call Allegheny County Police at 412-473-1300.

The rampage represents the latest mass shooting to rock the United States, where such attacks have become commonplace.

Late last month, a factory worker killed three people and wounded 17 in a shooting spree in Kansas.

Other recent mass shootings include a rampage by an Uber driver that left six people dead in Michigan and the December terror attack in San Bernardino, California, in which 14 people lost their lives.