The first threat came by phone: Several pipe bombs had been placed throughout Zoo Miami, home to giraffes, rhinoceroses, alligators and African elephants. People armed with AR-15s, the caller warned, would be stationing themselves around the zoo’s perimeter.

The zoo immediately evacuated all guests to safety, followed by employees once they had secured the animals. Law enforcement officers arrived shortly after, swept the park and found nothing. The zoo was closed for the remainder of the day. Guests received refunds.

That was a Friday. Then on Saturday, someone emailed another threat. And again on Sunday. And once again on Monday.

The authorities believe all four days of threats in mid-March were prank calls meant to sow chaos, a tactic known as swatting.

“It was a bit of a nightmare,” Ron Magill, a conservation liaison for Zoo Miami, said of the string of calls. “We’ve never received anything like that and then to have it happen on four consecutive days was just mind blowing.”

It was part of a string of swatting incidents targeting zoos and aquariums across the United States starting this spring, about 40 in all, according to the Association for Zoos and Aquariums, with recent episodes this month in Orlando, Fla., and San Francisco. And those are only the ones the organization knows about.

Other targets have included zoos in Denver, Memphis, Louisville, Ky., and three in Ohio — Columbus, Akron and Toledo. No group of swatters has taken responsibility for the calls and emails, and the authorities see no apparent link to animal rights groups.

The threats have added zoos, meant to be happy places of diversion for children’s field trips and family vacations, to the real-world sites that have fallen victim to disruption by anonymous internet trolls. A long list of institutions and public figures in recent years have been targeted by similar hoax calls or messages: schools, hospitals and Walmarts and the homes and offices of politicians.

“I think the objective of these people is to be disruptive and to make people feel less safe and secure in their daily lives,” said Daniel Ashe, the president and chief executive of the Association for Zoos and Aquariums, which has about 250 members, including 210 in the United States.

“It’s one of those unfortunate aspects of modern life,” he said.

Zoos prepare for such events, and the public should feel it is safe to visit, Mr. Ashe said, noting that none of the swatting calls disrupted the care of animals. That meant the giraffes and rhinoceroses were brought into their overnight holding pens and fed, just a little earlier than normal.

“They’re always going to put the interest of the animals first,” he said, referring to the animals’ caretakers. “They’re going to make sure they’re secure.”

Mr. Ashe said the association has worked with the F.B.I. and Department of Homeland Security to help its members determine the credibility of threats and the appropriate response. He said he hoped that the coordinated efforts with local and federal law enforcement “will result in prosecution.”

After the hoax calls were made to the three Ohio zoos in May, the F.B.I.’s field office in Cleveland said that it was “aware of recent threats involving a number of zoos and other entities over the past several days” and that it was “working with our law enforcement partners to determine their credibility, share information and take appropriate investigative action.”

In response to the initial swatting attacks, the zoo and aquarium association held emergency webinars for members to hear about how zoos responded to the hoaxes

For Zoo Miami, the effects went far beyond ruined field trips.

Over the four days, the swatting threats cost the zoo tens of thousands of dollars in lost revenue and payment to the Miami-Dade Sheriff’s Office for a security detail. The zoo also spent $1 million to update its security cameras. It had hoped to make another long-term investment — its own bomb-sniffing dogs — but Miami-Dade County only approved a detection system for concealed weapons.

The zoo has also started checking bag at the entrance and added new protocols to make sure staff work areas are secure.

“It pains me because this is a family institution, this is where you come with your kids,” Mr. Magill, the zoo’s conservation liaison, said. “We just want to make sure everybody is safe, we want to be sure people aren’t bringing in things that might hurt the animals.”

Mr. Magill said the animals were secured during the evacuations as if it were closing time at the end of the day with plenty of food and water.

The animals didn’t seem to be aware that anything was amiss. It’s like “a very slow day at the park,” Mr. Magill said, in contrast to preparing for a hurricane, when animals experience stress from disrupted routines.

Mr. Magill said the zoo wasn’t able to pinpoint the origin of the calls or subsequent emails that were used for the threats. “They were very sophisticated” and went through a series of untraceable I.P. addresses, he said.

Keven Hendricks, a cybercrime expert with the National White Collar Crime Center who teaches law enforcement officers how to investigate swatting, said swatters “do this for the LOLs.”

“This is what they find funny,” Mr. Hendricks said. “This is them finding camaraderie.”

Groups of swatters target public settings because doing so almost always leads to coverage on the local news, Mr. Hendricks said. The groups then use the news coverage to create hype videos “to add to their bravado online,” he said.

The groups largely congregate on apps like Discord and Telegram. Mr. Hendricks guessed that multiple swatters worked in tandem to make the threats across the country, though he hadn’t seen anyone take credit for the zoo swats.

Mr. Hendricks urged law enforcement to pursue prosecutions.

“It is a perfect storm because most state and local partners will not pursue these cases,” Mr. Hendricks said. “That only emboldens these actors even more.”

The San Francisco Zoo and Gardens prepared for the scenario that so many other zoos had encountered. In addition to routine drills, the zoo connected with state and local authorities and alerted their staff that there was a high probability of a swatting incident. They put a plan into place.

Sure enough, on June 8, a caller said shooters were going to arrive at the zoo.

“Our main goal was to shelter in place so police could sweep the area,” said Lamar Harris-Walker, a vice president at the zoo. Some visitors waited in service areas for the animals that were out of public view.

About an hour later, the zoo was cleared and returned to normal operations.