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.”