Newtown, Connecticut, put its school system on lockdown on Monday afternoon after a threatening call was placed to an elementary school about a mile from the site of a mass shooting six months ago.
The Hawley school, one of Newtown's four elementary schools, received a phone call shortly after 2 p.m. ET on Monday that contained "an implied threat to the Hawley staff and students," Superintendent John Reed said in an email message to parents.
No one was injured and no threat was found after the brief closure.
The incident came as families prepare to mark the six-month anniversary of the December 14 shooting at Sandy Hook Elementary School, in which a gunman killed 20 first graders and six adults before turning his gun on himself. The shooting inflamed a national discussion about school security and gun rights.
Monday's incident, which prompted officials to lock the doors to Newtown schools, delayed buses bringing students home by about 40 minutes but caused no other disruptions.
Newtown police and the Connecticut branch of the Federal Bureau of Investigations are investigating the incident, "as well as others that we have received in past months," Reed said.
An FBI spokesman did not immediately respond to a request for comment.
(Reporting by Edith Honan; Editing by Scott Malone and James Dalgleish)