Catalan, her American collegue Michael Sharp and their Congolese interpreter Betu Shintela were killed on March 12th while gathering evidence of mass graves in the Kasai region.
A UN board of inquiry said in a new report that they were likely assassinated by militia members from the area.
In a separate report last month, UN experts said they could not rule out the involvement of state security forces.
Sweden on Thursday joined the US, Britain, France and Japan to support UN Secretary-General Antonio Guterres in launching an international follow-up inquiry to establish criminal responsibility.
There are already ongoing criminal investigations in Sweden, the DRC and the United States.
“The UN's board of inquiry is not enough. The work is not finished as long as the perpetrators walk free. Further UN action is needed in support of the ongoing national criminal investigations in Sweden, the US and the DRC,” said Swedish Foreign Minister Margot Wallström in a statement.
Congolese authorities have already arrested nine people in connection with the murders, but some countries and human rights groups have expressed concern that key suspects have not been caught.
“Accountability has yet to materialize,” the Reuters news agency quoted Britain's deputy permanent representative Jonathan Allen as saying. “Those who ordered their killings remain at large.”
The region has seen a major spike in violence since September when government forces killed tribal chief and militia leader Kamwina Nsapu who had rebelled against President Joseph Kabila.
The unrest has claimed more than 400 lives and forced more than 1.2 million from their homes, according to UN figures. Unconfirmed local statistics put the number of dead as high as 3,000.