A wave of threats directed at schools across the nation forced authorities to lock down buildings or evacuate students.

The threats, which appear to be driven by automated calls, have been directed at elementary, middle and high schools in states across at least three time zones.

Some districts reported that the calls included a bomb threat while others just described the calls as "threatening."

The calls were reported in Washington, Oregon, Iowa, Colorado, Delaware, Florida, Maryland, Minnesota, Montana, New Hampshire, New York, Pennsylvania, Vermont and Wisconsin.

At least two schools in southwest Washington - Sunset Elementary in the Cheney School District and Greywolf Elementary in Clallam County - confirmed they received the calls. In Oregon, Portland Public Schools said they received similar kind of calls at their schools.

Lee Allinger, superintendent of schools in Appleton, Wis., said he can't remember a string of threats like this that seem to be connected. Janet Berry Elementary School in Appleton received an automated threat at about 11:45 a.m.

"It's the first one that I can recall that's been of this nature, where it's been an automated call or a robo kind of call and that it's been widespread — not just next door to us but across the state and the country," he said.

Tom McCarthy, a Wisconsin Department of Public Instruction spokesperson, said the department did not know how many schools statewide had received threats.

The Federal Bureau of Investigation responded to request for comment, writing, "We are aware of recent bomb threats at various schools in different states, and we remain in touch with our law enforcement partners to provide assistance if needed. As always, we encourage the public to remain vigilant and to promptly report suspicious activities which could represent a threat to public safety."

In the United Kingdom, at least 21 schools received bomb threats as well, according to The Independent. The Daily Mail reported 26 schools were closed.

The USA TODAY NETWORK has not confirmed that any of the threats are connected.

Colorado

Cherokee Trail Elementary (Parker): Students were evacuated Monday afternoon after “anonymous threats to schools around the state,” the Parker Police Department said in a tweet. Parents and caregivers were able to pick up their children, and some streets around the school were closed off.

Delaware

Bomb threats were reported at six Delaware schools.

— East Millsboro Elementary School: Police learned at 11:09 a.m. that a man had called the school and said there was a bomb inside. Beacon Middle School in Lewes,Polytech High School in Woodside and Stanton Middle School in Stanton received similar calls within the next half hour.

— Automated bomb threats were also reported at Mispillion Elementary School andSunnyside Elementary School earlier in the morning.

Iowa

Altoona: The school received a bomb threat at 11:54 a.m. saying bombs were inside the school. Students were relocated to a local church, where parents were able to pick them up. Buses will run Monday for parents who cannot pick up their children.

Valley High School (West Des Moines): The school received a bomb threat Monday afternoon. Students who could drive were allowed to go home while students who take the school bus were taken to a safe location until buses could take them home.

Tate High School (Iowa City)

Excelsior Middle School (Marion): The school was immediately evacuated after receiving a bomb threat about 12:30 p.m. The school dismissed students to parents and guardians.

Montana

North Middle School: The school was evacuated after receiving a bomb threat. Students were bused to an elementary school, where parents could pick them up.

New Hampshire

Portsmouth High School: The school was evacuated after it received a bomb threat. Police cleared the building and students were allowed back inside. No bomb was found.

Fuller Elementary School (Keene): The school was evacuated after receiving a bomb threat Monday morning.

Vermont

Northeast Elementary School (Rutland): The school was evacuated after staff received a bomb threat Monday morning. Students were taken to another school and parents were notified.

Wisconsin

Janet Berry Elementary School (Appleton): The school received a short, anonymous, automated phone call at 11:47 a.m. indicating a bomb would go off in the building. Staff and students stayed outside as police searched the school. They were allowed to return at 1:40 p.m. Superintendent Lee Allinger said district leaders decided not to send children home after consulting with the police. The playground at Berry is large enough that educators could keep children at a safe distance from the school. Had the threat happened at a school with a smaller playground, students would have been sent home.

Oakwood Elementary School (Town of Algoma): The school received a threatening phone call around 11:50 a.m. School officials immediately implemented a "soft lockdown" and began evacuating the school, sending staff and 473 students to the Oshkosh YMCA. Parents were asked to pick up their children at the YMCA.

Edison Middle School (Green Bay): The school was locked down for about an hour Monday afternoon after receiving a threatening automated call at 11:40 a.m. Green Bay police expect federal investigators to take over any additional investigation.

Pleasant Prairie Elementary School (Kenosha): Students and staff were evacuated after an 11:30 a.m. bomb threat. They were taken to a middle school, where parents could pick them up.