Policemen block the site of the stabbing happened on the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong) 
 

 Parents wait outside the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)

People watch from a window of the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)

Parents wait outside the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)

Students withdraw from the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)

Parents speaks with a policeman outside the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)

Police vehicles wait outside the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)

Students withdraw from the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)

Police vehicles wait outside the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)

A policeman evacuates vehicles outside the Spring High School campus in Houston area, the United States, Sept. 4, 2013. A 17-year-old student was stabbed dead and three others were injured on Wednesday in an attack at the high school in the U.S. state of Texas, local media reported. (Xinhua/Song Qiong)