The victims, all from the nomadic Muslim Peuhl or Fulani community, were reportedly killed within hours of being arrested in the northern town of Djibo on April 9, HRW said in a statement.
The mother of an ISIS recruit has told Euronews she believes her son is still alive - three years since reports he was killed in Syria.
"It was truly a murder attempt. There was blood everywhere," a trade union representative for prison guards said.
Muthana, a 24-year-old Alabama woman, made a desperate plea to U.S. officials earlier this week to be allowed to return home.
Trump has told a gathering of foreign ministers at the State Department that the so-called Islamic State could be completely defeated by next week.
Men were killed in series of co.ordinated raids in Giza and the Sinai.
The weekend visit will also include talks with President Idriss Deby.
Islamic militants are again being blamed for an attack on the Indonesian police after four men were shot dead and one arrested after killing one police officer and wounding two.
Family dies in attack but eight-year-old child survives, while at least one police officer dies and several other victims are wounded a day after an Islamic militant family is blamed for bombing three churches in country's second city, killing 13.
And they reveal that he was known to be a possible threat to national security.