The three arrests reported Thursday for threats of mass killings bring the total to at least 30 people detained on similar charges since the shootings in El Paso, Texas, and Dayton, Ohio, earlier this month.

Even in a country where such attacks have become a fact of life – there have been 263 mass shootings in the U.S. this year, according to the Gun Violence Archive, which defines them as four or more people wounded or killed – those numbers are staggering.

What’s behind them? Not all assaults with multiple fatalities are alike, but experts on gun violence have warned about the “contagion effect’’ spurred by the media attention given mass shootings, which often prompt copycat incidents.

READ MORE: US: Serial killer who targeted older gay men executed

In addition, the back-to-back massacres in El Paso and Dayton, less than a week after a gunman killed three and injured 12 at a festival in Gilroy, California, have put both authorities and regular folks on heightened alert. Not all the threats are serious, but police have little choice but to treat them as such.

Why, why, why? Weeks since Gilroy, El Paso and Dayton: What if motives behind mass shootings never emerge?

"I think people are on edge and there’s more concern in communities, more concern among police," said Vanderbilt University professor Jonathan Metzl, who teaches sociology and psychiatry.

Law enforcement officials have asked the public to report suspicious activities, which are often noticed but not reported by peers and relatives of potential assailants.

“It’s possible that members of the public are doing better at reporting warning signs to law enforcement,’’ said University of Alabama criminology professor Adam Lankford. “Although the ‘If You See Something, Say Something’ campaign has been in place for many years, more Americans may now realize that it applies to mass shooting prevention, not only terrorism prevention.’’ Demonstrators demanded stricter gun-control measures in Dayton, Ohio, days after nine people died in a shooting there.

READ MORE: El Paso gunman confessed to targeting Mexicans, police say

Demonstrators demanded stricter gun-control measures in Dayton, Ohio, days after nine people died in a shooting there. 

Mr Lankford and Mr Metzl point out threats of mass violence typically surge after notorious incidents like last year’s onslaught at Marjory Stoneman Douglas High School in Parkland, Florida, which left 17 dead.

In his research for the book “Dying of Whiteness,’’ Mr Metzl said he also found an increase in instances of white men parading around in public with semiautomatic weapons and protective gear in states that allow open carry. He said the country’s current political climate has emboldened them to do so.

"I think there are two phenomena going around right now,’’ Mr Metzl said. “One is more awareness of possible warning signs for mass shooters, and the other is a kind of acting out, particularly by white men, demonstrating that open carry is a kind of performance of white masculinity.’’

Here’s a tally of the arrests made for threats of mass murder since the Aug. 3-4 carnage in Texas and Ohio.

The list is presented in alphabetical order by state and is not meant to be all-encompassing.

Arizona

• Aug. 16: Tempe police arrested a 14-year-old boy after his threatening messages toward a school were discovered online.

• Aug. 13: Brian Thomas Keck, 35, a transient in the Phoenix area, is arrested after threatening to blow up a local Army recruiting center.

California

• Aug. 20: Rodolfo Montoya, 37, a hotel cook who was unhappy at work, was booked on assault weapons charges and accused of making a criminal threat after authorities said he told a co-worker he intended to shoot fellow employees and guests at the Long Beach Marriott. Police say they found several firearms, an assault rifle, tactical gear, dozens of high-capacity magazines and hundreds of bullets at his home.

• Aug. 15: A 15-year-old girl was arrested after threatening on social media to shoot up a Fresno high school, posting a photo of a store display case with rifles inside and the message, “Don’t come to school tomorrow.’’ Connecticut • Aug. 15: Brandon Wagshol, 22, was arrested on four counts of illegal possession of large-capacity magazines after an investigation from Norwalk police and the FBI. Police said Wagshol had displayed online "his interest in committing a mass shooting."

Florida • Aug. 22: A 16-year-old girl faces a felony charge of making a written threat to kill when, annoyed by the chat alerts from her sister’s phone, she grabbed it and wrote, “Next person to say something is the first person I will shoot on the school shooting that will take place this Friday.” Several parents alerted officials at St. Mary Magdalen Catholic School, and even though authorities determined the girl did not plan to carry out the threat, her actions were deemed illegal.

• Aug. 20: Eric Lin, 35, a resident of Clarksburg, Maryland, was charged with making online threatening communications “to injure and kill a South Florida resident and to kill all Hispanics in Miami and other places,’’ according to the U.S. Attorney’s Office for the Southern District of Florida.

• Aug. 16: Tristan Scott Wix, 25, of Daytona Beach was charged with making threats to commit a mass shooting after police were alerted to several texts he allegedly sent, one of them saying, “A school is a weak target.. id be more likely to open fire on a large crowd of people from over 3 miles away.. I'd wanna break a world record for longest confirmed kill ever."

• Aug. 16: A 15-year-old boy is arrested in front of his mother at their Volusia County house after posting a message on a video game chat that read, "I Dalton Barnhart (a fictitious name) vow to bring my fathers m15 to school and kill 7 people at a minimum." In a body camera video posted by the sheriff’s office, the mother’s teen is heard defending him and saying, "He’s still a little boy. He's not one of them crazy people out there doing stuff. He shouldn't be treated as though he's a terrorist or something because he made a silly statement on a stupid video game."

• Aug. 12: Anthony Reed, 33, was arrested and charged with a felony after threatening to bring a gun into a Walmart in Clearwater if the remote-control car he was purchasing didn’t work.

• Aug. 11: Miranda Perez, 28, was arrested in Palm Beach County and accused of making an online written threat to shoot up a school because she was unhappy her children were moved there.

• Aug. 9: Richard Clayton, 26, was arrested by Winter Park police, which said he posted a threatening message on Facebook that read, “3 more days of probation left then I get my AR-15 back. Don’t go to Walmart next week." He was charged with making a written threat to kill.

• Aug. 4: Wayne Lee Padgett, 31, was charged with making an active-shooter threat that forced the evacuation of a Walmart outside Tampa.

Hawaii

• Aug. 19: Nainoa Gazman Figueroa, 18, was apprehended in Maui and charged with first-degree terroristic threatening after tweeting that he “might shoot up a school’’ Illinois

• Aug. 16: Chicago authorities charged Farhan Sheikh, 19, with transmitting a threat in interstate commerce after he posted a message on the social media platform iFunny that said in part, “I will proceed to slaughter and murder any doctor, patient, or visitor” of a local abortion clinic.

Indiana

• Aug. 19: Thomas Matthew McVicker, 38, was detained in Indianapolis after making what the FBI called "credible threats to conduct a mass shooting and suicide." Officials say his intended target was a church in Memphis, Tennessee.

Michigan

 • Aug. 18: Former Marine Arnold Holmes, 33, faces three felony counts, including threatening domestic terrorism, after posting YouTube videos singling out a Ferris State University professor and northern Michigan hospitals. Relatives say Holmes served in Iraq and has PTSD.

Minnesota • Aug. 13: Police in Albert Lea arrested a 15-year-old girl for making an online threat to shoot the local high school.

Mississippi • Aug. 16: Two juveniles were charged with making text-message threats against two schools in Tupelo, forcing one of them to go on partial lockdown.

• Aug. 11: A 17-year-old boy in Lamar County was arrested for threatening Oak Grove High School on social media. Police said it was later discovered he did not have a weapon.

Missouri

• Aug. 8: Dmitriy Andreychenko, 20, was charged with making a terrorist threat after causing a panic by walking into a Walmart with a tactical rifle, a handgun and 100 rounds of ammunition, according to Springfield police. "I wanted to know if that Walmart honored the Second Amendment,’’ police said he told them.

Nevada

• Aug. 9: Conor Climo, 23, a security guard in Las Vegas, was charged in federal court with possessing bomb-making material, which authorities said he discussed using to attack a synagogue. Climo has links to a white supremacist group and also targeted an LGBTQ bar, police said.

Ohio

• Aug. 16: James Patrick Reardon, 20, an acknowledged white nationalist, was charged with telecommunications harassment and aggravated menacing after police in Cleveland received a tip that Reardon identified himself as the shooter in a Jewish community center that had not yet occurred. Authorities said they found ammunition, semi-automatic weapons and anti-Semitic information at his parents’ home.

• Aug. 12: Justin Olsen, 18, was arrested with threatening to assault a federal law enforcement agent, and authorities found close to 10,000 rounds of ammunition along with assault-type weapons and shotguns in his home. The FBI also said it found extremist postings linked to him.

Oklahoma

• Aug. 18: Mark Anthony Dietrich, 18, was charged with violating the Oklahoma Computer Crimes Act after making an online threat to murder the families of Claremore police officers who had being called the previous day to intervene in a disturbance.

South Dakota

• Aug. 19: Daniel Nazarchuk, 37, was arrested for making Facebook threats to blow up the Rapid City Police Department and the Pennington County Sheriff’s Office. He also faces drug and property damage charges.

Texas

• Aug. 8: A 13-year-old boy was taken to a juvenile detention facility on charges of making a terroristic threat after his Instagram posting prompted a Walmart in Weslaco to shut down. “The Weslaco Police Department reminds the public, especially juveniles active on social media, that all threats will be investigated and taken seriously; there are consequences for these threats!’’ a department release said.

• Aug. 10: Police in Harlingen performed a “terroristic threat arrest’’ of a man they did not name who they believe was planning to go on a shooting rampage at a South Texas Walmart.

West Virginia

• Aug. 12: Nathan Clark, 25, was arrested at his home in Charles Town and charged with making terrorist threats. Police said they found PVC pipes and guns in his house.

Wisconsin

• Aug. 22: Kevin Pinkham, 31, a distribution center employee in Eau Claire, was charged with making terrorist threats after telling a co-worker he planned to “shoot up the place.’’ Officials aid they found four rifles and hundreds of rounds of ammunition in his home, as well as a pistol and ammunition in his car.

This story was first published by our American sister paper, USA Today