An Ethiopian soldier shot a man dead in front of several people after his phone rang during a public meeting, Amnesty International said.
It is one of many incidents the rights watchdog recorded from a security crackdown in Oromia regional state at the end of 2018 and 2019.
Ariti Shununde, 32, was killed during operations to suppress an armed group, the Oromo Liberation Army (OLA), according to witnesses quoted by Amnesty International.
The army called for a public meeting in one local area in Oromia and collected all the phones of those who had turned up.
One of the phones then rang and when soldiers asked for the owner to identify himself Mr Ariti stepped forward. He was then shot in the back twice, the witness added.
His family was told to bury Mr Ariti immediately.
Amnesty says it has found evidence of the extrajudicial killings of 39 people in Oromia, including Mr Ariti. Through witness testimony, it details how three other victims were taken out of police cells and shot dead.
In late December 2018 soldiers killed 13 people in the town of Finchawa in what Amnesty International describes as indiscriminate shooting.
The security forces are also accused of rounding up thousands of people they believed were supporters of the OLA.
The OLA is a breakaway faction of the Oromo Liberation Front (OLF), a former separatist rebel group which laid down arms following peace talks with Mr Abiy.
The Oromos, despite being Ethiopia’s largest ethnic group, had long complained that they were marginalised from political and economic power.
A wave of protests in Oromia preceded the appointment of Mr Abiy – who is Oromo himself – as prime minister in April 2018.
He introduced a number of reforms, including recognising the OLF, that helped the country move away from its more oppressive past.
But many in the state continued to feel excluded and disquiet did not go away. Protests sprang up and opposition groups began to gain support.
In general, Mr Abiy has said he wants to foster a sense of national unity in the face of ethnic divisions, but also wants to celebrate that diversity.
His efforts to walk this tight-rope have created difficulties.
This was the year that Prime Minister Abiy Ahmed won the Nobel Peace Prize.