Update 5.15pm: A gunman has died after a shooting at a high school in Maryland which left two people injured, a sheriff said.
Two students, male and female, were wounded in the shooting in a hallway at Great Mills High School, St Mary's County sheriff Tim Cameron said.
The gunman died after a school resource officer fired at him, Mr Cameron said.
The officer and the gunman both fired a round and it was not known if the officer's bullet killed the suspect, he added.
The school resource officer is unhurt.
The male student was armed with a handgun when he wounded two others just before classes began, Mr Cameron said.
All three were taken to hospital and the gunman was later pronounced dead, he added.
The school's principal told parents last month that it had investigated threats of a possible shooting and found they were "not substantiated".
Update 3.20pm: Shooter among three injured at Maryland school
Three people have been injured in a shooting at a high school in Maryland, including the perpetrator, a sheriff's spokeswoman said.
There were no fatalities in the shooting at Great Mills High School, St Mary's County Sheriff's spokeswoman Cpl Julie Yingling said.
Cpl Yingling said the two victims were shot, but she does not know how the shooter was injured.
The three injured were all taken to hospital.
The principal told parents last month that the school had investigated threats of a possible shooting and found they were "not substantiated".
Jake Heibel told parents that school officials interviewed two students in February who were overheard mentioning a school shooting and found that they posed no threat.
His letter was posted on local news site The Bay Net.
But Mr Heibel said the school still increased its security after social media posts about a possible school shooting "circulated quite extensively".
Earlier: Injuries reported after shooting at school in Maryland
Injuries have been reported after a shooting at a high school in Maryland, a sheriff's spokeswoman said.
St Mary's County sheriff's spokeswoman Cpl Julie Yingling said there are injuries at Great Mills High School but she did not know how many or the severity of the injuries.
She also said she had no information about fatalities.
Agents from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives were heading to the scene.
The county sheriff said deputies are on the scene and that parents or guardians should stay away, going instead to Leonardtown High School to reunite with Great Mills students there.
Maryland governor Larry Hogan and Representative Steny Hoyer tweeted that they are monitoring reports and urged people to follow the instructions of local law enforcement at the scene.
Here’s a look from the scene outside Great Mills High School in Great Mills, Maryland, which is currently on lockdown after a shooting at the school, St. Mary's County Public Schools says https://t.co/O0tm9C2l8X pic.twitter.com/2dJ05pghF6— CNN (@CNN) March 20, 2018
According to CNN, there have been 19 confirmed school shootings this year in the US prior to today.
-PA and Digital Desk