Police in Northern Virginia and New Jersey are investigating bomb threats that were called in to dozens of schools Friday morning, threats that prompted evacuations and lockdowns.

Many of the schools received the threats via automated phone calls — known as robocalls — a method that has become increasingly common for school bomb threats nationwide and one that is difficult to track. A rash of robocalls led to evacuations and lockdowns of 13 schools in three states in January, none of which were found to be credible.

At least seven schools in Northern Virginia received bomb threats Friday morning, prompting some to evacuate and others to lock down. Falls Church City’s lone high school, George Mason High, was evacuated after it received what school officials described as an “automated bomb threat” by phone shortly before noon.

Fairfax County Police are investigating bomb threats that were called in to three public schools and one private school between 11:22 a.m. and noon, but authorities declined to say whether those threats were from robocalls. Police determined them not to be credible and Fairfax County Public Schools officials decided to continue class normally at the three high schools that received threats.

“Police are investigating and have determined the threats are not credible, and are intended only to disrupt school operations,” said Mary Shaw, a school system spokeswoman. “We do not believe any FCPS students are at risk and we are continuing with normal school operations at all of our schools for the remainder of the day.”

Police also are investigating threats called in to a Catholic high school in Arlington and Stone Bridge High School in Loudoun County, both of which evacuated.

Bomb threats also were called into schools in a dozen districts in New Jersey at around 11 a.m. Friday, disrupting school for thousands of students, according to a report in

The problem has become so severe that the Bergen County Prosecutor’s Office has decided to host a symposium to discuss how to handle such threats. It was the second time in a week that robocall bomb threats shut down multiple schools in New Jersey.

The entire school district in Augusta, Maine also shut down Friday as police investigated a threat that was emailed to a high school.

Robocalls are becoming an increasingly common method of delivery for school bomb threats, said Amy Klinger, an assistant professor at Ashland University in Ohio and a co-founder of the Educator’s School Safety Network, a national non-profit school safety organization.

Klinger said that Internet-based organizations charged the equivalent of $50 in bitcoins to create a bomb scare using automated phone calls, which account for 13 percent of all threats, according to her school security research.

“Schools are really caught in this dilemma of what do we do? Do we ignore it? But you can’t,” Klinger said, noting that some schools receive multiple threats in a single day and evacuate their buildings for each occurrence, creating significant delays during the academic day. “That’s a really dangerous precedent to say we’re just going to stop responding. So it’s really kind of a Catch 22 that schools have found themselves in. We need to respond but every time we do it just generates more threats.”

Klinger, in a recent school security report, wrote that bomb threats against schools have increased significantly in recent years. So far during the 2015-2016 school year, Klinger found that a total of 745 bomb threats had been made against schools, a 143 percent increase compared to the same time period during the 2012-2013 school year.

In January, schools nationwide received 206 bomb threats, the highest number ever recorded, Klinger found. Her research also determined that threats were made indiscriminately, with 48 of the 50 states in the country recording school-based bomb threats.

“It’s not going away,” Klinger said. “The only option is to empower schools to be able to handle these things.”