(Reuters) - The number of calls to an FBI tip line designed to head off mass shootings and other attacks surged by 70% in the week after twin massacres in El Paso, Texas, and Dayton, Ohio, federal officials said on Monday.

FILE PHOTO: FBI police vehicles sit parked outside of the J. Edgar Hoover Federal Bureau of Investigation Building in Washington, U.S., February 1, 2018. REUTERS/Jim Bourg

The Federal Bureau of Investigation fielded more than 38,000 phone and online tips during the week after the shootings on the first weekend of August, up from the 22,000 tips it typically receives on a weekly basis.

The surge is evidence of an America public made jittery by a steady drumbeat of mass shootings.

“Such increases are often observed after major incidents,” the FBI said in a statement. “As always, the FBI encourages the public to remain vigilant and report any and all suspicious activity to law enforcement immediately.”

FBI officials said the number of tips the center receives each week fluctuates and not all are actionable. Some tips turn into FBI investigations while others are forwarded to local authorities. The number of FBI tips also does not include the thousands of tips that state and local law enforcement agencies have received since the shootings.

According to a report published by the National Threat Assessment Center earlier this year, three out of every four perpetrators in 55 mass shootings and attacks in 2017 and 2018 made prior threats or sent messages that sparked concern.

More than a half a dozen people have been arrested and charged across the United States in recent weeks as a result of tips, according to media and police.

They have included an employee at a Wisconsin distribution center who called police after a coworker threatened to carry out a workplace shooting and an Alabama resident who alerted authorities after his friend, a Florida trucker, sent him messages about his plans for a shooting at a Memphis church.

In Michigan, a person told authorities that a former classmate threatened to shoot 200 police officers during a phone and text conversation, while a woman in Florida told police that her ex-boyfriend texted her about his plan to kill 100 people in a mass shooting, according to police.

“The general public are definitely taking these more seriously,” said John Mina, the sheriff in Orange County, Florida and on the board of directors of the International Association of Chiefs of Police.

Mina is no stranger to mass shootings. He was the police chief in Orlando, Florida, the night of June 12, 2016, when a gunman opened fire at the Pulse nightclub, killing 49 and wounding 53.

In addition to tips resulting in arrests, law enforcement agencies in the U.S. have taken more than 20 people into custody for threats made on social media since the El Paso and Dayton shootings, according to media and police.

Mina said there has been an increasing amount of resources devoted in local law enforcement agencies to access tips and threats found online, even when the person may not be serious about carrying out the attack.

“Law enforcement has always acted upon it. The difference is now we are being a little more vocal about it,” he said. “Agencies are pushing the message out. It’s not a joke. We are going to arrest you.”