A school security guard spotted them on video: Four teenagers gathered in a high-school hallway, one with what looked like a rifle slung over one shoulder.
He alerted police, and within minutes Monday morning, scores of emergency vehicles screeched into Upper Dublin High School's parking lot as the school went into lockdown. A few students tweeted mild panic, and some parents raced to rescue their children from dangers unknown.
But it was just an umbrella.
Police tracked the students seen on the security camera to their classrooms, where they found the long umbrella and were told that the teens' suspicious body language was part of a school science project to simulate the human immune system protecting the body.
While the scare at the Montgomery County high school was unusual, the unease that sparked it wasn't. Students returned to schools in the region Monday to find new or reinforced security measures in place in the wake of Friday's carnage at a Connecticut elementary school, where a man gunned down 20 children and six adults before killing himself.
Police patrolled some schools, mostly to provide reassurance to skittish students and parents, educators said. Schools brought in counselors to talk with students or held brief assemblies or informal classroom chats.
"Every day, the principal greets each child warmly at the front doors. Today, each child got an extra long hug from her," said Sudi Southall, the mother of a first-grader and fourth-grader from Pennington, N.J.
Many schools sent home reminders about locked doors and visitors' policies or even restricted access further.
"Previously, they would just buzz you in because they have a visual of you at the door, and wave you to wherever you need to go. Now they're actually intercomming and verifying who are who you are, even though they know me, and you actually have to sign in," said Filomena Sannes, a mother of two from Broomall whose 7-year-old son attends Russell Elementary in the Marple-Newtown School District.
Some parents found such changes futile, albeit understandable.
"It is really not an issue of school safety - how could anyone prevent someone from blasting (their way into a) school? It is a school after all, not a prison," Southall said. "I think that the issue is a gun issue, and it makes me mad that parents and citizens of this great country cannot unite and change the laws that are so flawed."
Brynn Warminsky of Narberth couldn't help but wonder about the possibility of copycats.
"I'm never that person who's so freaked out or anxious, but I was playing with the idea of: 'Do I really want to send him to school today?'" said Warminsky, whose 6-year-old son attends kindergarten at Merion Elementary School.