Ten people were killed -- nine students and a teacher -- in a shooting at Texas' Santa Fe High School Friday morning, a law enforcement official said.\r\nSeveral others were injured, including two law enforcement officers, the official said.\r\nPipe bombs and pressure cookers were found at the scene, according to the official.\r\nWitnesses described students running from the school as they heard gunshots; they also described hearing an alarm at the school, though the sequence of events wasn't immediately clear.\r\nAn armed person walked into an art class at the school and began firing what looked like a shotgun, a witness told\u00a0CNN affiliate KTRK.\r\nThe witness told KTRK she saw a girl shot in the leg.\r\nAt least 12 people from the school are being treated at three hospitals, authorities said. Seven people are at Clear Lake Regional Medical in Webster, Texas; two patients are being treated at Mainland Medical Center in Texas City and three patients are at John Sealy University of Texas Medical Branch in Galveston.\r\nAt least one police officer was injured, Gonzalez said.\r\nAerial video from the scene showed several police officers outside the school. Some were searching students and their backpacks.\r\n\r\nWitnesses describe hearing an alarm and gunfire\r\nAngelica Martinez, a 14-year-old student, told CNN she and her schoolmates were being evacuated at one point "like it's a fire drill."\r\n"We were all standing (outside), but not even five minutes later, we started hearing gunshots," she said. "And then everybody starts running, but like the teachers are telling us to stay put, but we're all just running away."\r\n"I didn't see anybody shooting, but like (the gunshots) were kind of spaced," Angelica said, adding she heard about four shots.\r\nThe witness who spoke to KTRK also said she heard an alarm. She didn't specify if that was before or after the gunfire she described in the art class.\r\nShe said she couldn't describe the shooter.\r\n"I didn't look. I just ran," she said.\r\nAnother student, Dakota Shrader, told\u00a0CNN affiliate KPRC\u00a0that she heard gunshots only after hearing an alarm in the school.\r\n"I was in the history hallway, and as soon as we heard the alarms, everybody just started leaving following the same procedure as ... (a) practice fire drill," Shrader said, breaking into tears. "And next thing you know, we just hear ... three gunshots, loud explosions, and all the teachers are telling us to run."\r\nThe entire school district is on lockdown, district officials said.\r\nThe high school has about 1,400 students, according to GreatSchools.org.\r\nThree shootings in eight days\r\nFriday's shooting was the third at a US school in recent days.