Nine people, including two young children, have been found shot dead in their house in the Russian-occupied eastern Ukrainian town of Volnovakha.
Ukrainian officials say they believe Russian soldiers killed the whole of the Kapkanets family on 27 October for refusing to give them their house.
Russian investigators say two male suspects have been held, saying they are Russian soldiers from the Far East.
Ukraine and Russia have begun separate investigations into the attack.
Photographs have emerged on social media showing blood-splattered and bullet-riddled bodies lying in beds, some of them still locked in an embrace.
Ukrainian ombudsman Dmytro Lubinets said on Monday that "Russians' bloodied hands were involved" in the Volnovakha killings.
"According to preliminary information, the occupiers have killed the whole of the Kapkanets family, who were celebrating a birthday and who had refused to hand their own house over to occupiers from Chechnya," he said on Telegram.
The Ukrainian-controlled Donetsk prosecutor's office, citing preliminary information, said the attackers in army uniforms shot the family dead after their demand to vacate the house had been rejected.
Two children, born in 2014 and 2018, were among the victims, the office added.
Russia's official Investigations Committee described those arrested as Russian soldiers from the country's Far East, who had signed contracts with the Russian military.
"According to preliminary information, the motive for the crime was a domestic conflict," the committee said in a statement.
Astra, a Russian news channel on Telegram, earlier quoted the victims' neighbours as blaming the Russian military for the killings.
"All of the neighbours are saying that the killers were in the military. We're all scared," one neighbour told Astra.
Baza, another Russian Telegram channel, said members of the household had had "a conflict with unidentified men wearing military fatigues with no insignia".
Volnovakha was captured by Russian forces just weeks after the start of the full-scale invasion of Ukraine in February 2022.
Most of the town has been destroyed.
The killings in Volnovakha come as fierce fighting continues on the vast battle front of Ukraine's eastern and southern regions.
Kyiv launched its counter-offensive in June, trying to split the occupying Russian troops in the southern Kherson and Zaporizhzhia regions in two, and make their resupply more difficult.
So far, the Ukrainian advance has been slow and costly, as the Russians have had time to prepare their defensive lines.
In recent weeks, Moscow has also been on the attack near the key city of Avdiivka in the Donetsk region, as well as Kupyansk in the north-eastern Kharkiv region.
Ukraine's military says all Moscow's attacks have been "unsuccessful", reporting heavy Russian casualties around Avdiivka - a gateway to the Russian-held regional capital of Donetsk.
The claims by the warring sides have not been independently verified.