ARLINGTON, Va. – Nearly two dozen schools in New Jersey and Virginia received bomb threats by robocall last week, prompting many to evacuate or impose a lockdown while police searched the buildings.

NBC Washington reports seven Northern Virginia schools received bomb threats by a robocall round mid-day Friday, and authorities are investigating whether they’re related to similar calls in New Jersey.

MORE NEWS: Know These Before Moving From Cyprus To The UK

Similar bomb threat robocalls were made to 15 schools in three New Jersey counties – Exxex, Bergen and Passaic – the same day around 11 a.m., Pix 11 reports.

For nine Bergen County school districts, it was the second Friday in a row.

The brief and unspecific calls featured a computer-generated voice claiming there is a bomb in the building. Many schools evacuated students, while others simply went about normal operations as police conducted searches. Several schools also went into lockdown, keeping students in classrooms while K9s searched for explosives. Still more schools let students go home early.

Amy Klinger, spokeswoman for Educators Schools Safety Network, conducts school safety training and said bomb threats are becoming increasingly common across the country.

“Unfortunately, it’s pretty much business as usual. That’s what we’ve been seeing almost every school day this whole academic year,” she told NBC Washington. “We’re averaging nationally somewhere between eight to 10 bomb threats a day.”

She said many school officials are growing frustrated by the constant interruptions.

MORE NEWS: How to prepare for face-to-face classes

“We look at threats and think oh, okay well maybe nothing’s probably going to happen. But the problem is schools are really in this dilemma of you have to do something. We can’t ignore it,” Klinger said. “But yet, we’re continually evacuating and so it really becomes a frustrating experience for schools.”

Students and police are also annoyed.

“Apparently, they’re all pranks. I don’t know why you would even think that’s funny,” an unidentified student told Pix 11. “So annoying. You’re just trying to get through the day and suddenly they tell us we’re being evacuated.”

Bergen County Sheriff’s Office spokesman Anthony Cureton pointed out “this is a disruption to education and resources have to be allocated that may be needed for a real emergency.”

The Washington Post reports the round of robocall bomb threats Friday are only the most recent in a trend of automated threats, and a similar rash of calls led to evacuations and lockdowns at 13 schools in January.

Rhode Island schools were among those that received the robocall bomb threats in January, and the Providence Journal tracked the calls to St. Petersburg, Russia.

Rhode Island State Police Col. Stephen O’Donnell told the Associated Press in February that Russian authorities do not provide the type of cooperation needed by investigators to track down those who placed the calls.