(Reuters) - Two men suspected of ambushing partygoers at a backyard barbecue near Pittsburgh and killing five of them, including a pregnant woman, were being sought by Pennsylvania police on Thursday.

“The murders were planned, calculated, brutal,” Allegheny County District Attorney Stephen Zappala Jr said. Three people were also wounded.

The Bureau of Alcohol, Tobacco, Firearms and Explosives said it was offering a $20,000 reward for information leading to the arrest and conviction of the gunmen.

One suspect armed with a .40 caliber handgun herded victims from behind the house toward an alleyway where a second suspect armed with “an AK-47 type” rifle shot them dead, Zappala said. Police recovered 49 bullet casings from two guns at the scene.

The shootings occurred in the working-class borough of Wilkinsburg in Allegheny County, which has a population of about 15,000 and is some 8 miles (13 km) east of Pittsburgh.

Zappala said the motive was not known, but authorities were exploring whether it was related to drugs.

“Everybody is a person of interest. There are some we are more interested in than others,” Allegheny County Police Superintendent Charles Moffatt said at a news conference, noting that the suspects had fled on foot.

This was the latest in a series of mass shootings in the United States that have fueled the debate over gun control and turned it into a presidential campaign issue.

The Allegheny County Medical Examiner’s office identified the dead as Brittany Powell, 27; her sister, Chanetta Powell, 25; Tina Shelton, 37; Shada Mahone, 26; and Powell’s brother Jerry Shelton, 35. Three of them were from the same family, Moffatt said.

Chanetta Powell was eight months pregnant, and the death of her fetus was ruled a sixth homicide under Pennsylvania law, Moffatt said.

Tina Shelton was a mother of five who held three jobs and was encouraged by family members to attend the barbecue to relax after work, her father Vernes Pugh and other relatives told local station WPXI-TV.

Two men were in critical condition, Moffatt said. A woman who was shot was treated and released from a hospital, Allegheny County spokeswoman Amie Downs said.