When Marlin Jackson arrived at his row on a Delta flight from Atlanta to San Diego in June, the middle seat was already occupied by a man with a sizable dog on his lap. Jackson squeezed by them to his window seat, and the Labrador mix lunged at his face. The attack lasted about 30 seconds, according to Jackson's attorney, and left him with facial wounds that required 28 stitches and scars that are still visible today.

The mauling, which Delta said was inflicted by a canine identified as an "emotional support" animal, was among the thousands of incidents that just pushed the nation's largest airline to tighten rules for passengers flying with service or comfort animals. In announcing the changes Friday, Delta said it flew 250,000 animals in those categories last year, an increase of 150 percent from 2015, while "incidents" such as biting or defecating had nearly doubled since 2016.

Delta's announcement emphasized safety concerns, but it also was spurred by a widespread perception among airlines and disability rights advocates that some fliers are fraudulently taking advantage of federal law to bring untrained pets of myriad species into crowded cabins.

Though the Americans With Disabilities Act defines service animals as trained dogs or miniature horses, airlines are bound by the more liberal Air Carrier Access Act of 1986, which allows free travel for "any animal" that is trained to assist a person with a disability or that provides emotional support. Airlines can require passengers with creatures in the second category to produce a letter from a physician or mental-health professional, but the documents are easily forged or obtained from websites that provide cursory, questionnaire-style "exams."

The result, airline officials complain, has been a surge in poorly trained animals that has turned some flights into airborne menageries, with dogs blocking beverage carts, cats urinating on seats and ducks wandering the aisles.

"It's created a real issue on our planes," said Taylor Garland, a spokesperson for the Association of Flight Attendants, which applauded Delta's changes. Garland said one union member was asked to administer oxygen to a dog that, according to its owner, was having anxiety midflight. Others have been bitten. "The aircraft cabin is a unique space, and … we need to recognize the limitations that exist when you're flying in the air in a metal tube."

Other airlines have not released their own figures, and the Department of Transportation says it does not collect data on service and support animals on U.S. flights. But the agency's reports on disability-related complaints show that those involving service animals nearly quadrupled between 2012 and 2016, when more than 2,300 were filed. Scrutiny of service animals is also sharpening on the ground: Nineteen states now have laws that criminalize passing off pets as service animals.

Airlines have pushed for new federal rules to reduce fraud, and the transportation agency plans to begin taking comments on proposed regulations in July.

But the outcry is not limited to airline officials. People with allergies to pet dander, who are also protected under federal disability laws, often think that their concerns are trumped by those of passengers with animals, said Sanaz Eftekhari of the Asthma and Allergy Foundation of America, which has started collecting stories from its members. And service-animal groups say that an increase in what are clearly pets on planes has led to heightened scrutiny of working animals - and even endangered some.

Gillian Lindt, 86, is blind and flies regularly with her guide dog between Washington, France and her main residence in Florida. She said she always requests a window seat so Stella's tail does not stretch into the aisle, and the 54-pound dog always wears a harness and sits quietly at her owner's feet.

On a recent flight, Lindt said, a woman sat next to her in the middle seat and plopped a small, barking dog onto the tray table. The woman said it was an emotional support animal and suggested that the two dogs could play together. Lindt was aghast.

"I'm trying to explain that, unfortunately, my dog would love to play, but they're trained not to, because this is work," she said. The woman was moved across the aisle, and an apologetic flight attendant wiped down the tray; the little dog barked on and off through the flight. "My dog knows she must never, ever bark when she's in a harness," Lindt added.

In 2016, the Transportation Department convened a panel of disability advocates and airline industry officials to propose new regulations on service animals, as well as on two disability-related issues relating to plane restrooms and in-flight entertainment. Several participants said they expected the animal topic to be the easiest to negotiate, but it was the only one on which the committee failed to reach consensus after nearly seven months.

Published documents show disagreement on many details. The airlines were hoping to align practices with the ADA by limiting permitted species. Some disability advocates suggested defining emotional support animals as only dogs and cats; others wanted to allow rabbits and household birds. Service-animal organizations wanted the department to recognize "psychiatric service animals" - typically dogs, which can be trained to perform tasks such as turning on lights for individuals with post-traumatic stress disorder - as working animals that don't require a medical letter.

Several participants backed tougher questioning at the time of ticket purchase to deter those trying to present pets as service or support animals - perhaps in part to avoid the travel charge of $100 or more they'd otherwise face.

"I'm not sure how big an effect it would have, but it keeps the honest people honest," said Jenine Stanley, the consumer relations coordinator for the Guide Dog Foundation for the Blind and America's VetDogs, who co-chaired the committee and praised Delta's revised rules.

The National Alliance on Mental Illness wanted any requirements to apply across the board to both service and support animals, said Angela Kimball, its national director of advocacy and public policy, who called the negotiations "so politically fraught." But the group did not want changes to create "undue burdens" that could prevent people with disabilities from traveling. Obtaining a letter from a mental-health professional can be expensive and time-consuming, Kimball noted.

"Under current regulations, there's a disparate response to emotional support animals … and we think that it's essentially a form of discrimination against a set of disabilities that are not visible," she said. "Any time you marginalize or create different conditions for a set of people, it's very disconcerting and stigmatizing."

Delta's new requirements, which take effect March 1, retain those distinctions. Passengers with trained service animals will need to submit a veterinary health form at least 48 hours before travel to the airline's new "Service Animal Support Desk." Customers with emotional-support animals or psychiatric service animals must do the same but also must provide a letter from a doctor or mental-health professional and a signed document saying the animal is trained to behave in public. (Delta also recently expanded its list of prohibited critters, including "farm poultry," hedgehogs and anything with tusks.)

The rise in emotional-support animals has coincided with growing publicity on the mental-health benefit of pets - an idea researchers say is poorly substantiated through studies but widely embraced by the public. Many owners say they, like service animal users, greatly depend on their emotional-support animals and face undue suspicion because of fakers.

Ashley Marie MacDonald, 29, says she doesn't mind producing a letter from her psychologist when she flies with her emotional-support parakeet, who stays in his cage. She has had anxiety, depression and a pain disorder since a work-related injury in 2012, and she doesn't want to be away from Stormy or "think about life without him." He comforts her when she is upset, she said, even licking tears from her face.

Last year, MacDonald recounted, an airline employee at a Florida airport questioned the validity of her letter at check-in and then kicked Stormy's cage, knocking him off his perch.

"I am very aware that there are people that go online and pay to have these forged documents, but I'm not one of them," said MacDonald, who lives in Cincinnati and said her disability forced her to stop working and end her pharmaceutical studies. "There should be a penalty against that."

Douglas Kidd, executive director of the National Association of Airline Passengers, said that much of the blame for the problems lies with Congress, which wrote too broad a law, and air carriers that have overbooked flights, reduced legroom and poorly treated animals that fly in the cargo bay. He argued that airlines should designate more spacious rows for passengers with true service or support animals.

"It's certainly a difficult situation to navigate," acknowledged J. Ross Massey, the lawyer Jackson hired soon after being mauled on that 2017 Delta flight. But in that instance, Massey said, the airline's middle-seat placement of a passenger traveling with a large dog was a "recipe for disaster." The 44-year-old Jackson, a government employee who lives near Mobile, Alabama, is now preparing for plastic surgery to correct some of the damage. He also is mulling legal action, according to Massey.