BRENDAN SMIALOWSKI via Getty Images Messages in chalk fill a brick wall where Heather Heyer was killed on Heather Heyer Way, at the Downtown Mall in Charlottesville, Va., on Aug. 11, 2018.

UPDATE: 2:05 p.m. ― Police on Friday arrested a 17-year-old male and charged him in a threat lodged on 4chan against Charlottesville High School.

Previously:

Public schools in Charlottesville, Virginia, were shuttered for a second day Friday as police investigated a social media threat to attack Charlottesville High School.

Investigators continued working to identify the person who threatened on the anonymous social site 4chan to launch an attack on the school.

Meanwhile, a separate threat was made on social media Thursday to attack Albemarle High School, just outside Charlottesville, according to the Daily Progress.

Police said Friday morning they had worked with the FBI to track down and arrest a teenager in that case, which they called an “isolated incident.” Albemarle County schools will see increased police presence on Friday, authorities said.

Officials ordered Charlottesville schools closed on Thursday after anonymous poster claiming to be a student at the high school threatened an “ethnic cleansing” there that would take place on Thursday, according to the message on 4chan reviewed by HuffPost. Other users goaded the poster, announcing, “School shooting tomorrow.” The messages reportedly were forwarded to local authorities on Wednesday evening.

It’s unclear whether the threat was legitimate, but Charlottesville City Public Schools closed schools out of caution, a police spokesperson said.

Jalane Schmidt, an associate professor of religious studies at the University of Virginia whose daughter attends the high school, told HuffPost that she received robocalls and emails from the district about the threat on Wednesday.

For her, an abundance of caution is just fine. Not only is the world still reeling from the shooting in New Zealand last week ― in which a white nationalist killed at least 50 worshipers at two mosques ― but Charlottesville has seen its own share of violent extremism, including the deadly “Unite the Right” rally in 2017. Both violent incidents were planned or advertised on sites like 4chan, 8chan and Reddit.

“We saw what happened during Unite the Right ― 4chan was a huge part of that,” Schmidt said. “I think they want to take precautions because they didn’t take it as seriously as they should have in 2017. This throws a monkey wrench into Charlottesville, but I’m fine with my daughter staying home today.”

It is maddening, however, that anonymous posters have the power to close schools in a community that is constantly on edge. Local teachers expressed anger on Twitter about having to explain to students that they live with such a threat.