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Five people were killed, including a pregnant woman, and three others were wounded after gunmen opened fire on a backyard party in Pennsylvania late Wednesday, with one of the shooters gunning down victims as they fled.

The gunmen are still on the loose after the shooting, which happened in Wilkinsburg, just east of Pittsburgh, at around 10:54 p.m., authorities said.

"The murders were planned, calculated, brutal," Allegheny County District Attorney Stephen Zappala said Thursday, reported NBC affiliate WPXI.

Four people — three women and a man — were found shot on a back porch and were pronounced dead at the scene, and a woman was pronounced dead at a local hospital.

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The Pittsburgh medical examiner's office identified the victims Thursday as: Jerry Shelton, 35; Tina Shelton, 37; Brittany Powell, 27; Chanetta Powell, 25; and Shada Mahone, 26. Jerry Shelton, Brittany Powell and Chanetta Powell are siblings, the medical examiner said.

One of the women killed was pregnant, Zappala said.

The shots were fired at victims from an alley in the back of the home, authorities said.

"It looks like, right now, they were all fleeing toward the back door of the residence when the second gunman fired from the side of the yard, and they all seemed to get caught on the back porch," Allegheny County police Lt. Andrew Schurman told reporters.

Three others were wounded. Two men were described as being in critical condition and a woman as stable. She was treated and released from the hospital.

Investigators found ballistic evidence from two weapons and believe at least two gunmen were involved, Schurman said. No one at the cookout or party fired any shots, police believe.

No suspects were in custody early Thursday. A motive was not immediately known.

A witness told the station that she heard around 20 shots ring out, followed by 6 more shots.

"Seeing that shakes people up; there’s little ones that live on this street,” Kayla Anderson told WPXI. She said Franklin Street is usually quiet at that time of night.

Wilkinsburg is a borough of around 15,000, east of Pittsburgh. The poor suburb is known for drug trafficking and gun violence, but neighbors told the Associated Press that the street where the shooting happened is generally quiet.