Investigators in Delaware, Maryland and Virginia are investigating a wave of bomb threats made to at least 13 schools on Monday morning — with many of the threats delivered via “robo-call” type automation.

There were no reports of injuries or bombs discovered. But schools were evacuated, some for several hours.

The disruptions came as, nationwide, agents are seeing an uptick in these kinds of threats. “More and more, they’re going to these automated calls,” said Michael Knight, a Bureau of Alcohol, Tobacco, Firearms and Explosives agent and spokesman for the agency’s division field office in Nashville, where agents have worked on a case of automated bomb threats to schools in Tennessee and Alabama.

Knight said the technology is similar to that used by marketing companies, in which the user can plug in phone numbers and make the same repeated calls.

“Sometimes, all they want to see is the law enforcement response, the thrill of it,” he said.

In other cases, those posing the bomb threats to schools are aiming to get students out of class at a time of day when they won’t have to return. Knight said ATF offices will help schools by using investigators, bomb-sniffing dogs and intelligence-sharing.

“Every bomb threat is taken seriously,” Knight said.

In Virginia’s Prince William County, a bomb threat was phoned in to Glenkirk Elementary School, in Gainesville, between 10:30 a.m. and 10:45, said Sgt. Jonathan Perok, spokesman for Prince William police.

Perok said the threat was made by robo-call and that it was not the first threat to a school made in such a manner. “We don’t get many but it has happened before,” Perok said.

Students were taken by bus to Marsteller Middle School in Bristow, Va., where they remained for the rest of the day, said Irene Cromer, spokeswoman for the Prince William public school system. “All of our schools have crisis plans, and evacuations are part of that,” she said.

A short time later, at 10:50 a.m., in St. Mary’s County, Md., Lexington Park Elementary received a call saying a bomb was set to explode in 30 seconds. Detectives say it is unclear whether the threat is related to similar incidents at other schools, said Jennifer Stone, spokeswoman for the St. Mary’s County Sheriff’s Office.

Stone said numerous police units responded, and no explosive devices were found inside or outside of the school.

At about 11 a.m., in Anne Arundel County, Tyler Heights Elementary in Annapolis received a bomb threat by telephone. Officials said it was a robo-call, with few specifics, but they decided it was best to evacuate the school and call 911.

Students spent about 20 minutes outside and then were allowed back into the annex and gymnasium, said district spokesman Bob Mosier. The disruption lasted about an hour.

“Fortunately, there was nothing to worry about, but you can’t take those chances,” Mosier said.

Threats were also reported at schools in the Maryland counties of Harford, Carroll, Wicomico and Talbot.

In Delaware, at least four schools received bomb threats between 9:30 and 10 a.m., Delaware state police officials said.

“A robotic style or computer generated voice phone call was received in each of the school’s offices,” the officials said in a statement.

State police investigators are probing the four incidents at H.B. DuPont Middle School, in Hockessin; Caesar Rodney High School, in Camden; Indian River High School, in Dagsboro; and Seaford Middle School, in Seaford.

As for the nature of the calls, “there are definitive similarities in all of them,” said Cpl. Gary Fournier of the Delaware state police. He declined to be specific about what was said. Officials said that investigators at three local Delaware jurisdictions also are investigating similar bomb threats.

Investigators there will work with counterparts in Maryland and Virginia to see what, if any, connections can be made, officials said.