A spokesman, Sirajuddin Ahmad, says Malala Yousufzai was warned three times to stop her activities promoting "Western thinking," but she did not.
He said the last warning was conveyed a week ago.
Ahmad said two of the attackers had expertise in shooting people in the head. The hit squad carefully examined the girl's route from school to her home.
The shooting of the teenage activist who spoke out in support of girls' education and against Taliban atrocities has horrified many in Pakistan and abroad.