SLOVYANSK, Ukraine — Forensic experts are excavating a mass grave that witnesses to the burial say could contain the bodies of as many as 20 people killed by pro-Moscow separatist fighters.

The digging began Thursday under a hot summer sun. A bulldozer clawed into a fresh mound of black dirt for several hours before finding the first traces of human remains. Then, as it removed layer after layer of soil, the stench of decaying flesh permeated the grassy area on the grounds of a children’s hospital.

“The whole thing happened in broad daylight,” said Irina, 62, a pensioner who said she saw the burial from her apartment. Witnesses said they saw a man and several armed rebels dig a deep hole on June 16 before tossing numerous corpses into the open pit. Then they piled soil atop of the bodies, leaving a foot-high mound.

“Two [Russian Orthodox priests] came and blessed them, even though that wasn’t their religion,” Irina said.

Two of the bodies in the grave are believed to be those of Ruvim and Albert Pavenko, ages 24 and 30. Sons of a local Protestant minister, they were kidnapped June 8 after attending a service at their church and then they were shot, according to Anton Gerashchenko, an aide to the minister of interior affairs.

Their abduction coincided with the occupation of Slovyansk by pro-Moscow rebels, who claimed the industrial city of 120,000 as part of the self-declared Donetsk People’s Republic.

Weeks of heavy fighting marked by artillery fire from both sides destroyed hundreds of homes and apartments and left several hundred people dead. Tens of thousands of residents abandoned the city, which the Ukrainian army recaptured on July 4. The rebels fled the city and are now concentrated in Donetsk, a city about 90 miles south of Slovyansk.

Ukrainian authorities believe the mass grave may also contain the bodies of local people who disappeared during the military action, or rebels themselves, who were either killed during the fighting or executed by order of the rebels’ military commander, Igor Strelkov. Once all the bodies are exhumed, they will be taken for identification.

Mixed in with the dirt removed by the bulldozer was at least one skull and several other human bones, believed to have been part of an old graveyard dating back to the Russian Revolution. There is an aging, concrete pinnacle-shaped memorial to those who fell during the Russian Civil War from 1917 to 1922 next to the excavation area.