WILKINSBURG, Pa. -- Police were seeking to identify suspects in a deadly ambush attack by two men who methodically shot and killed five people, including a pregnant woman whose fetus also died, and wounded three others, two critically, at a backyard cookout late Wednesday night.

Multiple law enforcement sources told CBS Pittsburgh the intended target of the gunmen may have been Lamont Powell, one of two men still hospitalized with gunshot wounds from the shooting spree. He is one of the victims in critical condition.

Sources say they are looking at the possibility the two gunmen were sent to Wilkinsburg to carry out what's being described as an alleged "revenge killing."

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Powell was allegedly involved in a shooting in Pittsburgh two years ago but was never charged. The shooting rampage, the sources said, might have been in retaliation for that incident.

However, Powell's mother told the station there is no way her son could have been involved in a homicide, and she has no idea why anyone would target him.

A $20,000 reward is being offered by the federal Bureau of Alcohol, Tobacco, Firearms & Explosives for information that leads to an arrest, CBS Pittsburgh added.

District Attorney Stephen Zappala said at a news conference Thursday the gunmen appeared to have targeted one or two of the victims, and drugs haven't been ruled out as a motive, said

"The murders were planned. They were calculated, brutal," Zappala said of the shootings.

Police have no suspects and a county executive urged witnesses to come forward.

Allegheny County Executive Rich Fitzgerald said providing any information "can be our first step to stopping the violence in our communities."

"As a community, we must say enough is enough," he said.

Four women, one of them eight months' pregnant, and a man were killed as they rushed toward the back porch to seek cover as a gunman fired a .40-caliber pistol at the partygoers who were playing cards and having a late-night cookout.

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. Two others were critically wounded.

The medical examiner officially ruled the death of the fetus a homicide Thursday afternoon, bringing the fatalities in the late Wednesday night ambush attack to six.

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.

Mike Jones, 57, lives in a duplex on a small hill overlooking the alley and backyard where the shooting occurred.

Although Wilkinsburg has a reputation for violence, Jones said it's rare in his neighborhood.

"This is unheard of," Jones said of the ambush, shaking his head as homicide detectives milled about in the yard and alley. "It doesn't happen around here."

The dead included three siblings, Brittany Powell, 27, who lived at the home; Jerry Shelton, 35; and Chanetta Powell, 25, who was eight months' pregnant. The other two were Shada Mahone, 26, and Tina Shelton, 37. All had multiple wounds and had been shot in the head.

"My whole family was massacred," said Jessica Shelton, the mother of the siblings and aunt of the other two killed.

"It doesn't make sense to take people's lives like that," said Shelton, who had been at the party earlier in the evening. She said one of the critically wounded victims is her son.

One of her grandchildren was at the party and saw his mother lying dead, then ran upstairs, Shelton said.

"He said he didn't want the bad men to get him," she said.

All of the victims were hit by shots from the rifle, and none from gunfire from the pistol, which "looked like a distraction almost," said ATF agent Chris Taylor. He said 49 shots were fired in total, 31 from the rifle.

"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," said Lt. Andrew Schurman of the Allegheny County homicide unit. "They all seemed to get caught on the back porch."

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 heard someone scream, "Mommy, Mommy!"

"It was terrible," the younger Morris said.