"I was saddened and angered to hear this [Tuesday] morning that Abdul Gani Lone was assassinated in Srinagar," Powell said in a statement.
"We have not seen any claim of responsibility for the murder, but this is a direct attack on hopes for a fair political process in Kashmir."
He said Lone's killers were clearly among those who opposed a peaceful, political resolution in Jammu and Kashmir.
"The United States condemns this crime in the strongest possible way. Mr Lone sought to achieve his goals through peaceful, democratic means and courageously stood up to extremists. We extend our condolences to Mr Lone's family and to the Kashmiri people," Powell added.
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