An investigation by medical charity Médecins Sans Frontières (MSF) into the "execution" of three of its workers during a humanitarian mission in Ethiopia's war-hit northern Tigray region has found evidence that the country's army was responsible for the killings four years ago.

MSF's report includes claims that Ethiopian troops were present at the scene of the killing of the three - a Spanish national and two Ethiopians.

"They were executed," MSF Spain's general director Raquel Ayora told the BBC. "They were facing their attackers [and] were shot at very close range... several times."

The BBC has asked the Ethiopian government for a response to the allegation.

MSF said it was releasing the findings of its report into the incident as the government had failed to provide a "credible account" of the deaths despite 20 face-to-face meetings over the last four years.

Thirty-five-year-old Spaniard María Hernández Matas, along with 32-year-old Yohannes Halefom Reda and 31-year-old Tedros Gebremariam, were killed on 24 June 2021 while travelling in central Tigray to assess medical needs.

"They were very professional and passionate," Ms Ayora told the BBC.

She added that the three were fully identifiable in MSF vests and their vehicle had the charity's flag and logos on either side when they were shot.

"So, they knew that they were killing humanitarian aid workers," she said.

Ms Matas had been working in Tigray since before the war and "was very much loved" by people in the region, Ms Ayora said.

Her death has been particularly devastating for her mother as she was her only child, the MSF official added.

Mr Tedros was killed soon after his wife had given birth to a baby girl. His widow named the baby Maria, after her father's killed Spanish colleague, Mr Ayora said.

The Tigray conflict broke out in 2020 following a massive fall-out between the regional and federal governments, with neighbouring Eritrea entering the war on the side of the Ethiopian National Defence Force (ENDF).

The conflict ended two years later following a peace deal brokered by the African Union (AU). Its envoy, former Nigerian President Olusegun Obasanjo, put the number of people who died in the conflict at around 600,000.

Researchers said the deaths were caused by fighting, starvation and a lack of health care.

MSF said the killing of its staff took place at a time when the conflict was intensifying, and Ethiopian and Eritrean troops were becoming increasingly hostile towards aid workers in the region.

The charity's report includes what it says is evidence that a convoy of soldiers from the Ethiopian army, retreating from fighting, was present at the scene of the deaths, which, it adds, is corroborated by satellite imagery.

The report says both civilian and military eyewitnesses had come forward to directly implicate Ethiopian army soldiers in the killings, including one who allegedly heard a commander order an attack on the aid workers' vehicle.

However the charity says "the level and nature" of the army's involvement in the attack "remains to be clarified".

"The review found a large body of corroborating evidence that placed a convoy of retreating ENDF troops on the road where the killings took place on the day of the incident," MSF said.