Moroccan authorities last month arrested Yassine Abaaoud, a younger brother of the suspected Islamic State mastermind of the Paris attacks, after he arrived in his father's hometown of Agadir, a Moroccan security source said Friday.
Abdelhamid Abaaoud, a Belgian militant of Moroccan origin, 28, is believed to have orchestrated last week's coordinated bombings and shootings that killed 129 people. He was among those later killed in a police raid in a Paris suburb.
Yassine was arrested last month after his plane landed in Agadir and has been held in custody since, a Moroccan security source, who declined to be named, told Reuters.
It was unclear why Abdelhamid's younger brother had traveled to Morocco or whether he had ties with the militant cell in Europe. The source declined to give further details on the arrest.
But Moroccan security officials had provided information that helped their French counterparts launch a raid in the Paris suburb of Saint-Denis on Wednesday, sources have said, where Abdelhamid was killed.
Morocco's King Mohammed VI was in France on Friday and met with President Francois Hollande, who thanked his visitor for the "effective help" given since the November 13 attacks in Paris.
A French police source said four representatives of the Moroccan security services were in Paris on Tuesday to meet the heads of judicial police.
Moroccan authorities have arrested scores of suspected Islamic State militants in recent months. On Monday, police detained four people linked to the group, the country's interior ministry said.
That cell was planning attacks using explosives, while its leader had close ties to Moroccans fighting with the Islamic State group in order to obtain logistic support. It did not give details of those contacts.
Around 1,500 Moroccan nationals are fighting with armed groups in Syria and Iraq, 220 have returned home and been jailed, and 286 have been killed, authorities said earlier this year.
Nearly 158 women and 135 children have also gone there.