Kate and Gerry McCann said it reflected progress in the investigation, adding they still hoped to be reunited with Madeleine, who went missing in 2007.
Portuguese prosecutors made Christian Brueckner an "arguido" on Thursday, but did not formally reveal his name.
Brueckner has not been charged and denies any involvement in the case.
Madeleine disappeared during a family holiday in Praia da Luz and investigators believe she was abducted from the apartment where the family were staying in the Algarve resort.
