(CNN) A disturbing trend is causing disruptions of school life across the country, forcing lockdowns and straining police resources, sowing fear and confusion among students, parents and educators, and tormenting already anxious communities.
The age of the active shooter has brought with it not only the frightful practice of mass casualty simulations in America's classrooms but also unsettling and potentially dangerous false active shooter reports, or hoaxes known as "swatting."
Nearly every week since the start of the academic year -- and months after a teen gunman killed 19 fourth graders and two teachers in Uvalde, Texas -- threats or false reports of active shooters have forced US schools from Maine to California to lockdown as law enforcement officers rushed to the scene. The FBI is working with state and local law enforcement on the investigations.
Authorities are still trying to determine who might be making the calls or if they're connected and part of a coordinated effort.
An unofficial tally by the National Association of School Resource Officers, based on news coverage, shows dozens of false reports of violence or armed person on campus that resulted in police responding in more than 30 states and the District of Columbia since September.
"Not only is this a criminal scenario but these are callous and inhumane acts against our kids, against our residents, against these parents," said Michael Sauschuck, public safety commissioner in Maine, where at least 10 schools received false reports of an active shooter on Tuesday.
Here's what we know about some recent swatting reports and the investigations:
The 911 caller, at about 8:30 a.m. on Tuesday, claimed to be a teacher at Sanford High School in Maine.
The caller reported an active shooter and injuries, according to Sanford Police Lt. Matthew Gagne. A description of the gunman was provided: A White male wearing a black jacket and pants and carrying a rifle.
The person who called police claimed to be locked in a staff room, police said.
At least nine other schools in Maine received "hoax-style" active shooter threats about the same time Tuesday morning, according to Sauschuck.
Sanford High got the first false report. Everyone knew what to do. Lights were shut off. Doors were barricaded. Students and staff hid behind desks and other furniture.
"I texted my mom saying that I love her because I didn't know how long it was going to take to get out of there or if we were all going to be OK," senior Paige Gauthier told CNN affiliate WGME-TV.
Two school resource officers were already in the building at the time of the call, Gagne said. Law enforcement personnel from various agencies rushed to the scene, searching the building and reporting no injuries. Or evidence of a shooter.
Students were safely evacuated to buses and transported to a nearby gym, police said.
"It was just a scary experience that I don't ever want to experience again," Melanie Sweetser, a parent, told WGME. "I would not want to wish this on my worst enemy."
In Portland, Maine, about 35 miles northeast of Sanford, police tweeted that they had responded to a 911 call of an active shooter at Portland High School. The school later tweeted that the threat was "believed to be a HOAX." Still, the building was on lockdown until police evacuated the students.
After it was determined the threatening calls to the Maine schools were hoaxes, Sauschuck told reporters that authorities were considering federal charges against those responsible for the false reports.
Sauschuck said dispatchers were "connecting the dots" after the threats -- based on the voice of the caller or callers, the phone numbers and the nature of the threats -- while crosschecking the information with law enforcement on the scene.
Gagne said the schools received similar calls and multiple agencies were investigating their origin. In Sanford alone, law enforcement dispatched three airlift units and at least a dozen ambulances.
"The price tag on these is substantial," Gagne said.
The FBI said in a statement it was working with state and local authorities "to gather, share, and act upon threat information" though it had "no information to indicate a specific and credible threat."
In Connecticut, a seemingly ordinary Thursday morning at Manchester High School was disrupted shortly before 9 o'clock by an anonymous email.
"I have a gun and ready to hurt people, plus explosives," read the email, according to an October 27 Facebook post from Manchester schools Superintendent Matt Geary.
School administrators, district officials and Manchester police held back on locking down given that about 20 Connecticut schools had received swatting calls the previous Friday, according to Geary.
At 9:42 a.m., another email repeated the threat and added, "I'll be entering the school at 11:16 AM." The school was immediately placed in a lockdown and additional officers were called, Geary wrote.
"No one was allowed in or out of the building and students remained in their secured and supervised rooms," according to the superintendent.
At 10 a.m., the school sent the first emails alerting parents and guardians of the lockdown.
"We quickly determined we would not be able to clear the MHS campus in a safe and orderly fashion by 11:16 a.m.," Geary wrote.
The lockdown lasted about two hours.
"We lost hours of instruction. And that has value," Jim Farrell, a Manchester Public Schools spokesman, told CNN this week.
"There's certainly some trauma. You have various kids who experienced trauma. You have some who are fragile. For kids to have to stay in a room for close to two hours. There's a price there. And then, sort of more macro, there's this whole desensitization ... of threats. Okay, what happens the next time?"
Geary praised "the calm and mature manner in which students acted." He thanked administrators, security staff, police and others who were there that day "for their rapid and thoughtful response to a difficult situation."
The previous Friday, October 21, also in Connecticut, Stamford High School was notified at 8:55 a.m. of an active shooter threat, according to Lauren Meyer, special assistant to Stamford's mayor. The school was put on lockdown. As police investigated the threat, other district schools went on lockdown.
Police quickly determined the initial report to be a hoax and, at 9:11 a.m., lockdowns in other district schools were lifted, Meyer said in a statement. The lockdown at Stamford High was lifted at 9:30 because of the number of first responders on the scene and the size of the campus.
Meyer said investigators believe the same person made similar false reports to schools in Westport and Bridgeport to the northeast.
In Westport, about 12 miles away from Stamford, police dispatchers received a report of an active shooter at Staples High School at about 9:10 a.m., according to Westport PD. First responders soon determined there was no shooter and the report was likely a "swatting call."
Still, police said, the school went on lockdown. Officers proceeded as if the threat was real. Every room in the school was checked. Classes resumed and officers were assigned to all Westport schools for the rest of the day.
Westport police and other agencies are still working to identify the caller. Lt. David Wolf said "it would definitely appear" the calls across the state which involved about 20 schools were connected, given their timing.
"These swatting hoaxes are dangerous on many different levels," Westport Police Lt. Eric Woods said in a statement. "The response to this type of call is immense and typically starts with the primary agency then multiple surrounding agencies. Each responding officer thinks they are responding to an active shooter incident and are in an emergency response mode."
Soon, police phone lines are swamped with calls from students, parents, reporters and others.
"This type of investigation takes up a significant amount of time and pulls many resources that would be better used elsewhere," Woods said. "The most harm is done to the community, where it creates a significant sense of fear and chaos."
At 10:17 a.m. on October 21, Windsor Locks Police -- about 77 miles north of Westport -- said dispatchers got a call from a man with an accent. The caller reported that 10 students had been shot at Windsor Locks Middle School.
Police responded and began clearing the school. Two nearby elementary schools went on lockdown, with additional officers sent to those schools as well as to the local high school, according to a police statement.
Windsor Locks Police Lt. Paul Cherniack said investigators were still trying to identify the caller but the phone numbers and voices are often digitally disguised. The voice of the caller is similar that of others who falsely reported active shooters that day, he said.
That same day in Enfield, just northeast of Windsor Locks, police received a report at 10:24 a.m. of an active shooter at Enfield High School, with multiple students injured. Officers secured the school, searched the building, and found no evidence of a shooter.
At 11:08 a.m., police in Norwich, in southeastern Connecticut, also got a call about an active shooter, at Norwich Technical High School, according to a department Facebook post. The school went on lockdown, officers responded and the report was determined to be false.
Cherniack noted that the funeral for two Bristol, Connecticut, police officers killed while responding to a call was held on October 21. Thousands of officers from the state attended the service, leaving many towns with skeleton crews.
"It seemed coordinated and it was all at the same time, one after another," he said of the swatting calls that day. "The callers seems to be similar (but) I can't confirm it was the same."
Charles Grady, an FBI community outreach specialist in Connecticut, said swatting incidents are common. The investigation is ongoing and there's "nothing to indicate" the calls are "all connected but there's nothing to indicate that they're not." Vetting the calls, he said, is a lengthy process.
"We can't take the chance that these swatting incidents are all manufactured to disrupt," Grady said. "We have to fully vet and determine the level of threat and the credibility of that threat."
Kate Dias, president of the Connecticut Education Association, in a statement called false active shooter reports "shocking, appalling, and downright dangerous."
"For Connecticut, these false incidents are extremely traumatic and painful and a vivid reminder of the shooting at Sandy Hook Elementary School that took 26 innocent lives nearly 10 years ago," Dias said.
The Sandy Hook Elementary School massacre in Newtown marks its 10th anniversary on December 14.
Dias noted that similar false reports have been received in schools in New Jersey, California, Florida, South Carolina and other states.
"We need to take all threats seriously to ensure the safety of our students, teachers, and communities and quickly put an end to the fear, danger, and disruption they create," she said.