Three people were injured and a high school was placed into lockdown during a shooting in Maryland.
Great Mills High School confirmed the incident and said it was "contained".
The Baltimore Sun said a county official confirmed three people had been injured and no one was killed.
The school is being evacuated, with pupils taken to a nearby school where a reunification centre has been set up.
A St Mary's County Public School spokesman said: "The building is orderly and the Sheriff's Office is conducting an investigation."
Maryland State Police confirmed the shooting, adding that the scene "is contained."
Mollie Davis, a student at the school, tweeted: "Hi Twitter. I am in Great Mills HS. My school is on a very real lockdown threat and there's already someone possibly dead. Please pray for us.
"You never think it'll be your school and then it is. Great Mills is a wonderful school and somewhere I am proud to go. Why us?"
She later added: "I am safe. The police came to my classroom and saw us to the bathroom and back and said we would be leaving soon."
St Mary's County sheriff's office told parents not to go to the school, but to report to nearby Leonardtown High School.
Jonathan Freese, a student at the school, spoke to CNN from inside his classroom. He said the incident happened around 8.00am (12.00pm GMT), almost as soon as school had started.
He said: "I've heard one person got hurt trying to get the gun.
"I'm a little shaken up. I didn't think it would really happen. It happened in the art hallway. I'm surprised I didn't (hear gunshots).
"Police are checking on students and seeing if they are safe and we are going to be escorted out. We are told to leave our stuff.
"The police have responded really quickly."
He said they had completed training for the event of a gunman in school.
He said: "I know I'm safe. Everyone responded, I heard sirens within five minutes."
The county public schools' tweeted: "There has been a Shooting at Great Mills High School. The school is on lock down the event is contained, the Sheriff's office is on the scene additional information to follow."
Maryland Governor Larry Hogan said: "We are closely monitoring the situation at Great Mills High School. @MDSP is in touch with local law enforcement and ready to provide support. Our prayers are with students, school personnel, and first responders."
We are closely monitoring the situation at Great Mills High School. @MDSP is in touch with local law enforcement and ready to provide support. Our prayers are with students, school personnel, and first responders.
— Governor Larry Hogan (@GovLarryHogan) March 20, 2018
According to The Baltimore Sun, St. Mary's Ryken High School is a private school about 15 minutes away from Great Mills.
The shooting comes four days before the planned March For Our Lives, organised after the Florida shooting in February, at Marjory Stoneman Douglas High School, where 17 people were killed.