To many parents, the school closings and the frenzied manhunt drove fears that Columbine still had the power to captivate would-be attackers and that the community would never be free from the massacre, which took place on April 20, 1999, and left 12 students and one teacher dead.

“What does this mean for tomorrow and the next day?” asked Dana Gutwein, 34, who has a first grader and third grader in the Jefferson County Public Schools District, which includes Columbine. Ms. Gutwein was unsure whether she would immediately let her children go back to class when the hundreds of schools that had closed on Wednesday reopen.

“I’ve felt like I’m on the verge of throwing up since this started,” she said.

Across the Denver metro area, parents struggled to find the right words to explain the latest safety warnings to children who have grown up in an anxious era of lockdowns and active-shooter drills. At Columbine, students who were organizing a day of service to mark the 20-year anniversary saw their preparations eclipsed by emergency text messages from the school district, news alerts on their phones and dread.

“This week our message was supposed to be about love and recommitment,” said Rachel Hill, 17, a senior at Columbine. “But now all the news surrounding Columbine is about fear.”

Some parents said they told their children to play close to home on Wednesday. Others shook their heads at how a threat from one teenager could keep half a million students home from school and throw an entire city into panic.