OSLO, Norway — The Norwegian police have concluded that footage showing the brutal killing of a Scandinavian tourist in Morocco by suspects who also filmed themselves pledging allegiance to the Islamic State is most likely authentic.Norway’s National Criminal Investigation Service, known as Kripos, said in a statement on Friday, “There is no concrete evidence indicating the video is not real.”It came as the Moroccan authorities announced the arrests of nine more people in connection with the killing of the university students Maren Ueland, 28, of Norway and Louisa Vesterager Jespersen, 24, of Denmark, in a remote corner of the Atlas Mountains. Morocco considers the killings a terrorist act.Thirteen men in all have been detained in the case. Morocco’s Central Bureau of Judicial Investigations said the nine new suspects had been arrested in the regions of Marrakesh, Essaouira, Sidi Bennour, Tangier and Chtouka Ait Baha. They were carrying arms and “suspicious materials” used in the manufacture of explosives at the time of their arrests. No further details were released.Ms. Jespersen and Ms. Ueland were attacked after setting out for a hike. Their bodies were found on Monday with stab wounds in their necks. Both women had been studying in Norway to become tour guides.On Friday, a plane took their bodies from Casablanca to Denmark. The Norwegian news agency NTB said Ms. Ueland’s autopsy would be performed in her home country.In the video of the killing posted on social media, one of the victims is shown screaming while a man cuts her neck with a knife. Kripos said in its statement that “some work remains on technical analysis and assessment” of the footage, and that the Norwegian police were working “closely with Danish police” on the case.Prime Minister Erna Solberg of Norway called the killings “a senseless attack,” and Prime Minister Lars Lokke Rasmussen of Denmark said on Thursday that the killings might be “politically motivated, and thereby an act of terror.”Four of the suspects had recorded another video one week before the killings in which they pledged allegiance to the Islamic State, according to Morocco’s general prosecutor.That video, which was shared on Twitter, showed the men vowing fealty to Abu Bakr al-Baghdadi and saying that they “cannot remain seated, witnessing the destruction caused by Crusader planes,” terminology typically used by the terrorist group to refer to Western airstrikes in Iraq and Syria.Kripos said that “neither Norway nor Denmark was mentioned in the video, nor was there anything specific about what action they should perform.”Investigators recovered surveillance footage showing three of the men putting up a tent near the victims’ tent in an isolated area of the High Atlas Mountains, an area popular with hikers. The same men are seen leaving the area after their deaths, according to Moroccan news reports.The Norwegian authorities said they were trying to trace the women’s activities before their departure for the village of Imlil. The women were found about six miles from the village.Morocco is generally considered safe for tourists but it has battled to rid some areas of Islamic extremists for years.In Norway, more than 400 people in the small town of Bryne held a parade and carried torches in remembrance of Ms. Ueland. On Facebook, Ms. Jespersen’s friends and relatives said they had received tens of thousands of messages from strangers expressing their grief, many apparently from Moroccans expressing shock and offering apologies.But some friends and relatives of the victims said they were also being sent the video of what the authorities believe to be the killing of one of the women.“We’ve been bombarded by many strange people,” Helle Jespersen, Ms. Jespersen’s mother, told a Danish newspaper. “Some were good — but others wrote that my daughter deserved to be dead.”Moroccans are also trying to stop people from spreading the video online. It is illegal in the country to publish or spread material that promotes terrorist activities.A Danish scholar on Islamic militants, Tore Refslund Hamming, said he had been approached by a person he believed to be American with a “fairly senior” position within the Islamic State who offered to share the suspects’ allegiance video online.“He asked if I wanted the video in advance and participate in hyping the video by sharing and liking it,” said Mr. Hamming, a doctoral student with the European University Institute in Florence.He said he declined, but that the video of the killing started spreading online half an hour later.A Danish Facebook manager, Martin Ruby, told Danish news outlets that the social media giant was helping the police to pull down the video “as quickly as we can.”“We do not allow content on our platforms to glorify or encourage terrorism or mass murder,” Mr. Ruby was quoted as saying by the news site DR.