Parents agonized over whether to send their kids to school and cautious educators locked down schools at the first hint of trouble on Monday, making for a nervous first day back to school since the massacre in Connecticut.
Schools 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. Many revised their security protocols, wondering if there was any way to keep a rampaging gunman from breaching their walls.
Safety was balanced against concerns over frightening children unnecessarily. Politicians debated whether to fortify schools with armed guards.
"It's hard to think about explaining to your kids something you don't understand yourself," said Karen Barbera, a mother of two in the San Francisco suburb of San Leandro who said she spent more time saying goodbye to them on Monday morning.
"I hugged them extra tight," she said. "Julian (her son) blows me kisses every morning, and I was sure to catch them."
School districts near Newtown, Connecticut, went into lockdown on Monday after a citizen reported a "suspicious person" at a train station near an elementary school in Ridgefield, about 20 miles (30 km) from Sandy Hook Elementary School, where a gunman shot dead 20 children and six adults.
Besides Ridgefield, at least two other nearby school districts also ordered lockdowns as a precaution, lifting them two hours later once police determined there was no threat.
"What's suspicious to one person may not be suspicious to another," said Lieutenant Jeff Kreitz of the Ridgefield Police Department, explaining what triggered three school districts to lock children in their classrooms. "It was just a precaution because of the situation at Newtown."
Some dangers were more serious.
Law enforcement reported arrests of people accused of making threats against public schools in Los Angeles, suburban Nashville, Tennessee, and the northern Indiana town of Cedar Lake, southeast of Chicago. All three were taken into custody on Sunday.
In Indiana, police jailed a 60-year-old man found to have concealed nearly 50 weapons at his home and accused of threatening to kill "as many people as he could" at a nearby elementary school.
Police and federal agents arrested a Los Angeles man they said had made criminal threats against "multiple elementary schools" on Facebook, and found a number of weapons at his home.
Authorities in Maury County, Tennessee, about 30 miles south of Nashville, arrested a 19-year-old man on suspicion of posting a Facebook threat to go on a Sandy Hook-style shooting rampage. He was arrested at his parents' home, where sheriff's deputies and FBI agents reported finding shotguns, a rifle, ammunition and a machete.
"Copycats do occur ... so we're taking threats very seriously," said Los Angeles-based FBI spokeswoman Laura Eimiller.
WHAT TO TELL CHILDREN
In Kernersville, North Carolina, first-grade teacher Molli Falgout struggled with how to address the tragedy with her young students - 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," said Falgout, whose pupils were the same age as those gunned down in Newtown, 6 or 7 years old.
In Fort Wayne, Indiana, the school district left it to teachers to decide what to say, rather than issue a blanket policy.
"We're trying to keeping things as normal as possible," spokeswoman Krista Stockman said.
At least one expert was clear. "Parents need to send their kids to school on Monday," said Kenneth Trump, president of the National School Safety and Security Services, a Cleveland-based consultancy. He said security was heightened at schools around the country.
A politician in Fort Lauderdale, Florida, is proposing police officers be posted at all elementary schools, suggesting the city re-assign police detectives and narcotics officers.
"We must find the necessary funding from the city and grants to provide protection at every elementary school," said Chuck Black, a city commission candidate.
Linda Cicero, and art teacher at Greenbriar Middle School in Parma, Ohio, liked the idea of extra security, at least for now.
"There was a police officer where the kids were getting dropped off (on Monday)," Cicero said. "It was a nice sense of security."
Our top photos from the last 24 hours.