Nawar Anwar al-Awlaki, known as Nora, was shot in the neck during the raid carried out by the Navy's SEAL Team 6 against an al Qaeda camp, according to NBC News.
"She was hit with a bullet in her neck and suffered for two hours," her grandfather Nasser al-Awlaki told Reuters. "Why kill children? This is the new administration. It's very sad, a big crime."
SEAL Chief Petty Officer William "Ryan" Owens was also killed during the hour-long gun battle, and three other American commandos were injured. An MV-22 helicopter that crash-landed had to be destroyed before the SEALs left.
"Almost everything went wrong," a senior US military official told NBC News of the operation, which was the first clandestine strike approved by President Donald Trump.
Born in New Mexico, the elder Awlaki spoke at the Capitol and the Pentagon after the 9/11 attacks, but eventually left the United States in 2002. The process of his radicalization accelerated after being imprisoned in Yemen - with US encouragement - and he went on to become a top recruiter and mentor to a number of al Qaeda operatives, including Fort Hood Shooter Nidal Malik Hasan and Umar Farouk Abdulmutallab, who tried to bring down an American airliner with explosives hidden in his underwear.
He was killed in a CIA Predator drone strike in 2011, the first time an American citizen was killed in such a way. Another US citizen, Samir Khan, who published the al Qaeda magazine Inspire, was also killed in the strike.
Roughly two weeks later, a US drone strike killed Awlaki's 16-year-old son, Abdulrahman. US officials denied he was their target.
Despite his death in 2011, Anwar al-Awlaki's fiery online video sermons have continued to inspire militants in the years since.
The death of 8-year-old Nawar will likely be used in militant propaganda efforts, especially since she is the second of Awlaki's children killed by the US. It was not immediately clear where she was born, but having an American father would have given her automatic dual citizenship for the US and the country of her birth.
"The perception will be that it's not enough to kill al-Awlaki," Karen Greenberg, director of Fordham University's Center on National Security, told NBC. "That the US had to kill the entire family."
According to Middle East Monitor, the US is already being blamed on social media for "assassinating children."