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Two men have been charged with homicide in a shooting that killed five people, including a pregnant woman, at a backyard cookout in Pennsylvania three months ago.

Authorities had been searching for two gunmen who opened fire just before 11 p.m. on March 9 at the gathering in the Pittsburgh suburb of Wilkinsburg, killing three women and one man at the scene. Another woman was pronounced dead at the hospital.

No charges had been filed until Thursday, when Allegheny County police charged 29-year-old Cheron Shelton and 27-year-old Robert Thomas with five counts of criminal homicide and one count of homicide of an unborn child. The two had been named suspects in April, according to NBC affiliate WPXI.

Cheron Shelton is escorted from the Allegheny County Police headquarters in Pittsburgh on June 23. AP

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The victims were identified by the Pittsburgh medical examiner's office in March as: Jerry Shelton, 35; Tina Shelton, 37; Brittany Powell, 27; Chanetta Powell, 25; and Shada Mahone, 26.

Related: Victims in Pennsylvania Party Shooting Included Pregnant Woman

Three others were wounded in the shooting, including one victim who was paralyzed and remains in a rehabilitation facility for his injuries, police said in a statement Thursday afternoon. The two others have since been released from the hospital.

Robert Thomas is escorted from the Allegheny County Police headquarters in Pittsburgh on June 23. AP

Police said in the statement that they received tips during their investigation suggesting the shooting was in retaliation for a previous murder.

Lamont Powell, one of the wounded victims at the cookout, had been a suspect in an October 2013 murder in Pittsburgh, police said. No one has been arrested in that killing and the case remains open.

Authorities believe Shelton and Thomas were looking for Powell and were tipped off by someone who told them over the phone that he was at a cookout.

After receiving the call, the two apparently went to the cookout and opened fire from an alley behind the home, continuing to shoot victims as they fled.

Police also charged Shelton and Thomas with three counts of aggravated assault, three counts of aggravated assault with a deadly weapon, six counts of recklessly endangering another person and one count of conspiracy to commit murder.

Jessica Shelton, whose three adult children and two adult nieces were killed, said at a news conference the day after the shooting that she had been at the cookout earlier and said everyone was playing cards and dominoes.

"It was a laughing cookout, a fun cookout," she said.

Victim Chanetta Powell was eight months pregnant with a baby boy and was in the process of narrowing down her list of names for him, Shelton added.