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Louisa Vesterager Jespersen (24) from Denmark, and Maren Ueland (28) from Norway were found dead early on December 17 near the village of Imlil in the Atlas Mountains.
The man arrested is also suspected of “involvement in recruiting Moroccan and sub-Saharan nationals to carry out terrorist plots in Morocco against foreign targets and security forces in order to take hold of their service weapons", the Central Bureau for Judicial Investigations (BCIJ) said.
It said also held Spanish nationality with residence in Morocco.
Nineteen other men have been arrested in connection with the case, including four main suspects who had pledged allegiance to Islamic State in a video made three days before the tourists' bodies were found.
Police and domestic intelligence spokesman Boubker Sabik this week described the four men as "lone wolves", and said "the crime was not coordinated with Islamic State".
Compared with other countries in North Africa, Morocco has been largely insulated from militant attacks.
The most recent took place in April 2011, when 17 people were killed in the bombing of a restaurant in Marrakech. In 2017 and 2018, Morocco said it dismantled 20 militant cells planning attacks in the country.