Fountain Green Elementary School near Bel Air was placed on lockdown shortly after noon on Monday after the school received what officials said was a "threatening phone call from an unknown sender."
Parents were notified of the lockdown by an automatic recorded telephone message from Harford County Public Schools around 12:15 p.m.
A follow-up call that went out about 20 minutes later said the school would remain on lockdown until dismissal and "appropriate personnel" would be present for the scheduled 3:30 p.m. dismissal.
The lockdown means nobody can enter or leave the school.
Field Day activities at the school, which began during the morning, were canceled for the afternoon, according to the message.
Law enforcement is investigating, the message said.
Harford County Public Schools referred questions to the Harford County Sheriff's Office.
Cristie Kahler, spokesperson for theSheriff's Office, said the agency was notified at 11:16 a.m. that the school had received a call about a threat made against students. She would not disclose the nature of the threat.
Kahler said around 1 p.m. that deputies determined the threat was "unfounded."
"We handle every threat as credible until it can be deemed otherwise," Kahler said.
Around 1:30 p.m., a small group of anxious parents was gathered under shade trees near one of the school parking lots, questioning each person who pulled up as they waited for dismissal.
LeeAnne Axelsson, of Bel Air, said she was in the middle of a day off from work when she received the first call about the lockdown.
"Of course, I jolted over here," she said.
Axelsson said she talked to a parent at the school who she said is an off-duty law enforcement officer.
"He pretty much assured us that everything was under control," she said.
Axelsson, who has a daughter in fourth grade and a son in third grade, was leaving when she received the second call and decided to stay until dismissal.
"I'll just stay here and wait until 3:30," she said.
Her youngest son, who is 5 years old and not in elementary school, came to Fountain Green with her. He sat under a tree and ate M&Ms – a treat from his mother – while they waited.
Axelsson also conferred with the other parents and Sheriff's Office deputies. She said six of her seven children have attended Fountain Green.
"They are top quality when it comes to the safety of the kids," she said of school administrators.
The school, off Route 543, has an enrollment of 570 students.