The assistant principal at the school says the alleged shooter has been taken into custody. The suspect appears to be a student.
Authorities say a second person has been detained.
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The shooting comes just two weeks before classes are released for the summer, administrators say.
Here's what we know so far.
7:40 a.m.: The initial calls come in about a report of shots fired at Santa Fe High School.
Between 7:45 a.m. - 8:00 a.m.: Reports circulate on social media of a shooting on the campus. Police are dispatched to the scene.
Students tell ABC13 that they were taken out of class around 7:45 a.m. They reported seeing a large police presence.
Another student reported that someone walked into an art class with a shotgun and started shooting.
Some parents also said that their children told them someone walked into an art class and began shooting. One parent says his son called him around 7:40 a.m.
8:13 a.m.: Santa Fe ISD issues a statement confirming that there was an active shooter on the campus. The school is put on lockdown.
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Statement from Santa Fe ISD:
This morning an incident occurred at the high school involving an active shooter. The district has initiated a lockdown. We will send out additional information as soon as it is available.
8:26 a.m.: Students tell Eyewitness News that they initially thought the incident was a fire alarm, but then they were told to run as fast as they could.
8:29 a.m.: Neighboring school districts including Dickinson ISD and Alvin ISD are put on lockdown as a precaution.
8:30 a.m.: Students are evacuated from Santa Fe High School.
8:32 a.m.: Special agents with the Bureau of Alcohol, Tobacco, Firearms and Explosives are responding to the scene.
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8:38 a.m.: The Harris County Sheriff's Office joins the investigation, saying it will assist with the response.
Between 8:45 a.m. - 8:50 a.m.: Multiple medical helicopters arrive at the school. The University of Texas Medical Branch confirms that it is prepared to accept patients and that patients are on the way to the hospital.
9:00 a.m.: Santa Fe High School Assistant Principal Dr. Cris Richardson confirms that an active shooter was inside the building. The person has been arrested.
"We have done an amazing job to get students out safely and reunite with parents as we are able to," she said.
9:05 a.m.: Santa Fe ISD says the situation is active but contained. There are confirmed injuries.
The statement said in part: Law enforcement will continue to secure the building and initiate all emergency management protocols to release and move students to another location. All other campuses are operating under their regular schedules. At this time, students from the high school are being transported by SFISD transportation to the Alamo Gym located at 13306 Highway 6. Parents may reunite with their students at this location.
Around 10:00 a.m. Law enforcement sources confirm that at least eight people are dead.
Harris County Sheriff Ed Gonzalez says that a second person has been detained. A police officer was injured and is being treated.
The extent of the officer's injuries are unknown.
10:45 a.m.: Sheriff Gonzalez updates the death toll, saying between 8-10 people were killed. He says they are students and adults.
11:18 a.m. Santa Fe ISD says that possible explosive devices were found at the school, which has been evacuated.
11:35 a.m. Officials in Sante Fe confirm that explosive devices were found in the school and the adjacent area. The community is urged to report any suspicious activity to 911.