Up to eight people have been reported dead in a shooting at a Texas high school, which is on lockdown.
One person is in custody after the attack at Santa Fe High School, which is about 40 miles (65km) south of Houston, according to school officials.
The school district has confirmed people were injured in an "active shooter" incident as classes began.
It is unclear if the attacker was a student. Police have not yet confirmed any fatalities.
The sheriff of Harris County, the largest county in Texas, tweeted that his officers were responding to a "multiple-casualty incident".
Sheriff Ed Gonzalez tweeted that one person was in custody and "a second one detained".
"An injured police officer is being treated, the extent of his injuries are unknown," he wrote.
Several students described hearing a fire alarm go off shortly before 08:00 local time. It is not clear how the alarm was activated.
One witness told KTRK-TV the shooting happened in her art class, and that one person shot was a girl.
"There was someone that walked in with a shotgun and started shooting," the student said, "and this girl got shot in the leg."
She said that she did not get a look at the shooter, because she ran to hide.
One 10th grader told networks she had an asthma attack while hiding in the woods.
News helicopters filmed students emptying their backpacks in front of armed officers in a field outside the school.
A bomb squad is currently on scene, and several helicopter ambulances have flown victims to a nearby hospital.
Federal officials with the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) are helping the investigation.