“A man accused of taking part in attacks in Mali in 2015 that killed dozens of people, including an American aid worker, was extradited to the United States to face multiple terrorism charges in federal court in Brooklyn, the authorities said on Saturday. The man, Fawaz Ould Ahmed Ould Ahemeid, 44, has already pleaded guilty to related offenses in Mali, where he was sentenced to death. He arrived in the United States on Friday and appeared on Saturday before a federal magistrate judge in Brooklyn, where he pleaded not guilty. Mr. Ahemeid was ordered detained pending trial. The terms of the extradition, and what it means for his sentence in Mali, were not immediately clear. In an indictment unsealed on Saturday, the U.S. Attorney’s Office for the Eastern District of New York charged Mr. Ahemeid with the murder of the American aid worker, Anita Ashok Datar, as well as providing material support to Al Qaeda in the Islamic Maghreb — Al Qaeda’s regional affiliate in North Africa — and related crimes. Ms. Datar, a 41-year-old public health expert from Takoma Park, Md., was a guest at the Radisson Blu hotel in Bamako, Mali, on Nov. 20, 2015, when two armed men attacked the hotel, killing 20 people. Prosecutors said Mr. Ahemeid masterminded the attack on behalf of the Al Qaeda affiliate and a local militant group, al-Murabitoun. According to the indictment, Mr. Ahemeid, a Mauritanian national, joined Al Qaeda in the Islamic Maghreb around 2007.”