Soldiers and park rangers are taking part in an operation to find two United Kingdom tourists kidnapped in the Democratic Republic of the Congo.
Ms Katumwa is understood to be the first female ranger to be killed in the park, which has been riven by violence in recent years.
Mr Johnson added: "My thoughts are now with the family of Virunga Park ranger Rachel Masika Baraka who was killed during the kidnapping, and with the injured driver and the released British nationals as they recover from this traumatic incident".
Boris Johnson didn't give any further details, but paid tribute to the authorities from the African country and the Congolese Institute for Nature Conservation "for their tireless help during this awful case".
Two British nationals kidnapped in Congo have been released, the UK Foreign Office has said.
The group's driver was injured and released shortly after the abduction, which took place near the village of Kibati, just north of Goma, on the morning of Friday May, 11.
He said: "We wish to extend our sincerest condolences to her family, and our thoughts are with all those affected by this incident".
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The incident came amidst the president's frustration over his administration's failure to fully fund the border wall . According to The New York Times , the president complained his administration has to do more on border security.
A park ranger who tried to protect them was shot dead by the attackers.
Congolese authorities are working with the Foreign Office to repatriate the British tourists, according to a park statement. The UNESCO World Heritage is home to around a quarter of the world's remaining mountain gorillas, and contains more mammals, reptile and bird species than any protected area on the African continent.
It spans (3,000 square miles 7,800 kilometres) along the border with Uganda and Rwanda.
But it is located in DR Congo's North Kivu province, where armed groups are fighting for control of territorial and natural resources, and poaching is a major threat.
Mai-Mai militia were blamed for an attack in which five rangers and a driver were killed in an ambush in the park.
The attack brought the number of rangers murdered in the park up to 175 in 20 years.