Cautious educators and police locked down schools at the first hint of trouble as nervous parents sent their kids back to school for the first time since the massacre at Sandy Hook Elementary School in Newtown, Connecticut.
Educators fearful of "copycats" or troublemakers who might phone in empty threats grappled with how to respond and whether to discuss Friday's shootings with children.
Safety was balanced against concerns over frightening children unnecessarily. Politicians debated whether to fortify schools with armed guards.
At least three school districts near Newtown went into lockdown on Monday after a citizen reported a "suspicious person" at a train station near an elementary school in Ridgefield, about 30km from Newtown, where a gunman shot dead 20 children and six adults at Sandy Hook Elementary.
"We had a report of a suspicious person at the Branchville train station, which is near the Branchville Elementary School. What's suspicious to one person may not be suspicious to another," said Lieutenant Jeff Kreitz of the Ridgefield Police Department.
Asked whether neighbouring school districts needed to go into lockdown, Kreitz said "it was just a precaution because of the situation at Newtown."
The lockdown was lifted about two hours later, police said.
In Redding, about 16km from Newtown, schools went into lockdown as a precautionary measure, police said.
The Katonah-Lewisboro School District in New York state, just over the state border from Ridgefield, sent parents an automated phone message notifying them district schools had been locked down because of an issue in a neighboring school district. It did not elaborate.
The Newtown massacre reverberated elsewhere in the country.
"We will not bring up the tragic events in lower grade classrooms," Cindy Wulbert, principal of Nettelhorst School in Chicago, said in a note to parents. "We will handle questions by addressing how they are safe at school and will ask younger children to talk with their parents."
In Kernersville, North Carolina, first-grade teacher Molli Falgout struggled with how to address the tragedy with her young students on Monday - and wondered if she should mention it at all.
"And if I do, what am I going to say about it? I'm just praying about it, because I don't know," Falgout said.
"Just letting them know that school is safe" is an important message, said, Falgout, whose pupils were the same age - 6 or 7 - as those gunned down in Newtown.
Falgout, 33, said she might think twice about opening the blinds all the way on her classroom's large windows as she normally does, worried that doing so would allow strangers to peer inside. "I'll kind of be on my toes more," she said.
Even schools on military bases were concerned about safety.
"Today, each school principal is reviewing, with the school staff, our established routine security measures and the procedures to be followed in the event of a school crisis," said Emily Marsh, superintendent for schools at the U.S. Army base Fort Bragg and U.S. Marine Corps base Camp Lejeune in North Carolina.
"Counselors, school psychologists, and our school social worker are available to assist teachers, students, and administrators," Marsh said in a note to parents.
In Florida, a politician in Fort Lauderdale is proposing police officers be posted at all elementary schools in the city.
Fort Lauderdale already provides school resource officers at its three high schools and five middle schools. The proposal would re-assign some city police detectives and narcotics officers to provide coverage at elementary schools and prevent copycats.
"We must find the necessary funding from the city and grants to provide protection at every elementary school. Public Safety has always been a primary concern for our city's residents," said Chuck Black, a city commission candidate.